As already said .... whatever size you want!
Some people have been having great success with jumbo sized blocks/designs lately.
Some prefer intricate and tiny, especially for babies.
Some don't like working with either ... and go for middle of the road!
Sometimes the scale of the print you are working with, will help determine the HST size.
Or if you happen to have precuts, then that may be your deciding factor.
If you are working with yardage, then you have lots of options!
If you have a layout/design you want to do, it can be done in any size of HSTs.
Just graph it out, and calculate how it will work out.
Then choose what suits you best.