Yes, it'll be A4 size (there's a template and everything).
The designs of most folders usually skew pretty cute and moe, yeah. I'm tempted to ask for one in your style just to be stand out but I dunno, lol. (For what it's worth, Moomin is crazy popular over here, as you might know... Mymble-ized Touhous???? jk
) If you don't want to do cute, like I said, something artsy/cool might work. If you want to brainstorm some ideas and send them to me, I can offer feedback. (also, you can do something interesting things with transparency effects).
I generally pay the commission fee in full upfront upon completion, although I'm open to other arrangements (buying stuff, exchanging your work with other artists, etc.). I just don't like to do a straight cut of the profits since that means I actually have to keep track of how many I sell, heh.