Technically, factory can make the size as big as possible, or as small as possible size of glass ball for Christmas tree decor.

But the regular size of glass ball for Christmas tree decor we normally see are:
2CM, 3CM, 4CM, 6CM, 7CM, 8CM, 10CM, 12CM, 15CM.

And 8cm, 6cm, 10cm are most hot sale glass ball for Christmas decorations.