Hi
The Apple Online Store includes details of wrist sizes that each combination of Apple Watch and band will fit.
For example, for the Space Gray Aluminum Case with Black Woven Nylon:
- Band with 38mm case fits 125–195mm wrists.
- Band with 42mm case fits 145–215mm wrists.
- http://www.apple.com/shop/buy-watch/apple-watch/space-gray-aluminum-black-woven- nylon?preSelect=false&product=MP052LL/A&…
The same information is also provided when buying additional watch bands as separate accessories:
http://www.apple.com/shop/watch/bands
In respect of choosing between case sizes, that is a matter of personal preference. Ideally, you might consider trying on Apple Watch at an Apple Retail Store or Apple Authorised Reseller in order to help you decide.