most bust what they use and found is not bigger than 2/3 of the screenheight,
this is also everyone personal reference to make it larger than that or smaller.
It also depends on the game you make and what fits best.
Best way, make 6 sizes of bust to test and play with it:
1/4, 2/4, 3/4, 4/4 and 1/3 and 2/3 (from 1 actor or random image)
see what looks best for you and confortable with, and create those
it's totally up to you how big, they are.