You'll probably want the smallest handheld that can shoulder the responsibilities you plan to give it, and sizes range from that of a credit card to the profile of a small notebook PC. The smallest handhelds-itty-bitty zingers that disappear into a pocket-are perfect for your calendar and contacts, the basic PIM functions. If you'll need to input extensive data or spend a lot of time reading the display, bulk up in size so that the full-size keyboard is more utilitarian and the larger display more clear.
The most popular size is the one that fits into the palm of your hand, and with it you can probably get most of the features and functionality you need. This handheld is still too small to include a keyboard, but surface-mounted buttons allow you to input data and enter commands, or you can tap the display with a stylus.
Larger handhelds come with increased computing power and a larger price tag, but they won't necessarily disappear into a pocket. These models will have more room for memory, expansion slots, a large display, and a keyboard that more closely resembles the one on your PC.