1.) Don't really care, preferably either the same height as DS/3DS/your GB cases or standard Genesis cases. Whatever the majority wants, I'll go with.
2.) Hold the game. Don't care about the manuals as I have none, and could easily keep them in a single case of some kind separately if I did.
3.) Black.
4.) Probably 20-30 I think. Having nice cases might encourage me to find more games, and thus get more cases.

Overall, just something simple and space saving would be nice.
Maybe someone who has GG, TG-16 and Wonderswan carts could throw up some comparison pics for you to see their relative sizes?