Is there a max size and is it configurable. I would like to see really big trees.

IIRC, there is a maximum width of 3x3 blocks and maximum height varies by species. A library could be made for even thicker trees but it would be very complicated.

I see, thank you. How about making the height configurable for really tall trees?

It depends. Most species don't grow tall because that is not their natural shape. If you like tall trees, large jungle, large spruce, spruce, and birch are the tallest species if I recall correctly.