(MONEY Magazine) – You might think that all technology moves in the same direction: smaller, faster, cheaper. But laptops have confounded this trend lately, managing to get bigger and smaller at the same time. You now can buy a wispy-thin ultralight weighing next to nothing or a backbreaking behemoth with an eye-popping wide-screen display. In between is such a range of sizes and capacities that shopping for a laptop has become like shopping for a new car: You've got your sleek sports models, your dependable Honda Accords and your gratuitously large SUVs.

If you're ready to step away from the world of safe, everyday mid-size laptops that make up the bulk of models sold, then one of these new lust-inducing numbers could be for you. But before you walk into CompUSA with a fistful of cash, it's important to know what you're getting into. Sure, it'd be nice to impress everyone with a cool new laptop, but if you're not careful, buying a superthin or ultralarge laptop can be the techno equivalent of buying a Ferrari or a Hummer during a mid-life crisis.

THE BIG NEWS

The biggest laptops have been getting bigger. A lot bigger. The largest models, often called "desktop replacements," sport 17-inch screens and exceed 11 pounds. They're so massive and heavy that they mock the very notion of portability (and being too big for most laps, they mock the appellation laptop as well). They are more like desktops that have the option of being carried in a bag if absolutely necessary.

Why even make a portable computer that isn't all that portable? Mostly to address the main complaints lobbed at laptops since their introduction in the 1980s--that they're not powerful enough and that their screens are too small to do much work on. The new 17-inch wide-screens from Toshiba, HP, Apple, Gateway and others will make sure no one raises those two issues again.

Besides, there's a certain segment of the buying public that insists on having it all. They want a portable computer that's as fast as a desktop and has a large screen and a big hard drive for their MP3 collections--even if 99% of the time it isn't going to do anything except sit on a desk--and they're willing to pay extra for the privilege. Not surprisingly, computer makers have no problem with this. They recognize that with our lives increasingly bound up in our computers, it's important for many people (myself included) to always have the option of taking their PC with them if they need to (if the house is on fire, for example).

But don't kid yourself into thinking you'll be lugging one of these to Starbucks every Sunday to work on that screenplay (you probably shouldn't kid yourself about that screenplay either, but that's for another column). This lack of easy portability, combined with a higher price, is why a 17-inch laptop is not the best option for most people. A laptop almost always costs more than a comparably equipped desktop PC; for what you'd pay for a 17-inch laptop-most cost more than $2,500--you could pick up a top-of-the-line desktop and still have money left over for both a digital camera and an MP3 player.

Knowing that these PCs are out of most consumers' price range, Apple, HP and others are pushing their 17-inch models to graphic designers, engineers and video editors, marketing them as portable multimedia machines that are just as powerful and productive as the desktops these professionals are typically chained to. As Apple loves to point out, a 17-inch PowerBook loaded with video-editing software like Final Cut Pro makes it possible to shoot and edit a film anywhere.

Toshiba and HP have created special versions of their 17-inch laptops, targeting people who want to watch movies, not make them. Both run Microsoft's new Windows XP Media Center software, which can record and play back TV shows much the way TiVo can (only without the monthly fee); it's even operated with a remote control instead of a mouse. They're hoping to sell the laptops to college students, cramped urban apartment dwellers or anyone who might not have the space for both a computer and a TV but still wants something to watch DVDs and TV on.

THE LITTLE THINGS

If there's one thing that's clear about these supersize laptops, it's that they're definitely not for the road warrior. More than a day of hauling one of these around in a shoulder bag could be enough to qualify anyone for disability. Frequent travelers know how much every pound counts and, fortunately, in recent years manufacturers have paid attention, coming out with ever smaller and lighter laptops.

An ultralight laptop is generally considered to weigh less than four pounds, and I never cease to marvel at how thin and light these things can get. Advances in screen technology, hard drives and processors have made it possible to squeeze more and more into less and less space, such that we've reached the point where you can cram all the power of a desktop from a couple of years ago into a laptop that's less than an inch thick and weighs no more than 2 1/2 pounds. The September issue of Vogue is bigger and more cumbersome.

While the truly hard-core business traveler might dispense with the laptop entirely and try to get all his or her work done on a PDA device like a Palm or a Pocket PC, I've found that nothing beats having an actual full-feature PC when it's time to get down to business. My Toshiba Portégé 2000 weighs in at a mere 2.6 pounds; I hardly even notice the thing when I'm carrying it. It's so light that I'll throw it in my bag even if there's a good chance I won't need it.

All that convenience and portability comes at a price. And not just a monetary price either. The battery life on almost all ultralights is uniformly dismal. Despite all the improvements in everything else that goes into a laptop, batteries remain stubbornly immune to the effects of Moore's Law, which holds that computers double in power every 18 months. Still too large and too heavy, overly bulky batteries continue to be the biggest obstacle to laptops getting even lighter and smaller.

Intel's new low-power Centrino technology has made it possible to eke out more life from laptop batteries. For the most part, though, don't expect to get more than a few hours of battery life out of an ultralight. Many PC makers offer additional extended-life batteries (my Toshiba came with one that attaches to the bottom of the laptop), but these almost always add extra heft and girth, defeating the purpose of getting something light and lean in the first place.

Besides battery life, manufacturers have had to make other sacrifices. Many ultralights, like Toshiba's R100 (the successor to my Portégé 2000), Dell's m300 and Sharp's MM10, lack internal optical drives, so if you want to play a DVD or even a CD you have to get an external CD or DVD drive, often at extra cost.

Smaller screens and keyboards are the norm. That's fine for a day or two, but after a week you'll be clamoring for a full-size keyboard and a monitor that doesn't require squinting. Sharp's MM10, one the tiniest ultralights of them all, is especially egregious, with a woefully small 10.4-inch screen and a keyboard that anyone over age 10 will have trouble typing on for long periods.

It all comes down to what you want and how badly you want it, since buying a laptop ultimately is all about compromises. You can have power, portability or price--pick two. Mid-size 13-, 14- and 15-inch laptops do the best job of balancing all these, but if you crave the massive power of a 17-incher or the sleekness of a featherweight ultralight, then go for it.

Peter Rojas is the editor of Gizmodo.com.