The Axim is a Pocket PC. If you carry your Axim around in your pocket (As I do), I suggest just getting the standard size. I also have two extended batteries I keep in my car for backup power. The mega is absolutely huge and in no way recommended.
If you have a case, the mega will not fit in most cases.
My recommendation:
Traveler: 1 standard + 1 extended
Average use: 2 standard
Heavy usage: 2 standard + 1 extended
As for the dimensions:
A rule of thumb for you-
The standard battery's depth is about the width of a HB2 pencil
The extended is about the width of two HB2 pencils (Obviously =P)
My extended batteries decided to disappear on me, so I cannot take a picture for you.
Brand- not the generic types. Not on Ebay.
Used/new- I usually get about 85% at full charge on a used battery (2-3 years). 100% on a brand new battery. Of course, used batteries are a lot cheaper.
Where to buy batteries- There is always the
classified section (Just post a WTB- Want to Buy).
Google is your friend.