I'd go with 10 - 6
and for how much to charge for the topper - how long would it take you to make? An hour, two? $10/$20? How much do you think your time is worth at an hourly rate?
i'd go with 6" - 10" too. i think the proportions look better with 4" difference in tier size than 2" (2" difference looks squatty to me).
as for the bear, i'd not charge less than $10, depending on how long it took probably $15.
adorable cake... i might have to steal it and change it to pink for an upcoming shower i'm doing.