Is cute yet realistic a thing? Haha. Probably halfway between realistic and cartoonish, if that makes any sense. Like, realistic enough that the anatomy is clearly defined. But whatever style you're comfortable with is fine!
No, for size, but large would be nice. (In the 800 pixel range, perhaps? Possibly 1000.)
And (don't laugh XD) what's dpi resolution? Is that like, psd, sai, etc? If so, psd should work fine ^^