The appeal of a PDA is in its size to performance ratio...such a small and convenient item to do such large and difficult tasks. However, the smaller they get the less they are capable of doing...the more cumbersome the size, the broader spectrum of capabilities. For example if you are looking for a small item to tuck in your pocketbook or jacket pocket for easy retrieval of clients' phone numbers or the living room rug dimensions, you can go small and not sacrifice on the tasks.
A PDA model with an easier to read display or keyboard will be naturally larger and heavier. If you are looking for digital camera functions and other high tech items, the PDA will again, tend to increase in physical size.
Decide what you need and what you would like to have and start from there.
And then there is size...
Although physical size is something to consider, the size of the PDA's memory and the speed of its chip are integral to usability and convenience. Just look at the changes in how we perceive computer speeds and RAM. You most likely will not be satisfied with a PDA that, due to lack of memory and speed, makes you wait for critical information. So, consider a PDA processing speed of 200 megahertz and memory of at least 32 megabytes with the capability of expanding. As usual, the faster the processor and the larger the memory, the higher the price tag.
Newer PDAs have an expansion slot for removable memory cards, enhancing the size of the PDA.