Smartphones come in two basic designs: Most common is the "candy bar" (pictured). Good for one-handed use. The other has a slide-out keyboard - bigger screen but more bulk. Good for images and anything that requires lots of typing.
Screen: Standard is 2.5 inches; 2.25 inches will do. Slide-outs can top three inches. Touchscreen is nice but not crucial.
Keyboard: All smartphones use a QWERTY layout, but all are not created equal. The devil is in the size, placement and texture of the keys. There's no substitute for picking up some phones and trying them out.