about 8" x 11" was the size my granddaughter's wanted for their tote bags. and they are firm believers you can never have too many bags ;) velcro is a good choice for closures, and shorter handles than what we would normally use...after all, they have short little arms, and don't usually like bags hitting below the knees :)
i made a little (change purse) size 'pouch' with velcro closure ... a little bigger than needed to hold business cards; put a ribbon on the corner and attached it to the inside pocket of the girl's tote bags so it can't get lost...it is where they stash their house key, library card, student id and any money they have to carry where ever they are going. they love their bags!