You can go too big - corners that touch in the middle are a square-in-a-square block, and that's made with a completely different method. But you almost can't go too small - sometimes a teeny pop of color in the corner is all a fabric needs to shine.
When you're saving the bonus HSTs, that can be part of the sizing. They will square 1" smaller than the snowball corner - my 4" corners layer cake snowball corners will yield a 3" unfinished HST.
And of course, there's the yardage. I chose 4" corners for my layer cake version because that cuts perfectly out of a yard of fabric! If I had gone up to 4.5" squares, it would require 1.25 yards, and I don't typically keep that on hand.
Week Two of the Layer Cake Snowballs
Make some snowballs!
Repeat 21 times!