I'm not sure the river bet on first hand should have been bigger. Maybe it can be argued that is should have been bigger, but you don't know how strong his flush is. I might have bet $38 (the pot).

If you know he has a K-high flush, then you could bet more and probably get a call. But as is, he might have a baby flush. I think part of this is just guess-work mixed in with a read on your opponent. I'd be happy knowing I bet an amount that he decided to call. But, I think a pot-sized bet would be good here.