na, the error your thinking of tman is max points on winding i think...

Make a worldspawn and add the following lines...
_blocksize 16384 16384 16384

This is the official answer from the authors....It does work but sometimes you need to open the worldspawn and highlight _blocksize 16384 16384 16384 and hit enter again !!

Then restart gtk ...load the map...and then vis.
Its a pain to start with but after a while you get used to it. ideally if you have done a long session in gtk then do the above before you exit gtk....restart and it should vis flawlessly

ahh.. didn't try the restart, but I'd tried the above hint,.. gotta remember to restart it..

I found (just a while ago actually) that one of the major problems I had was that I was trying to stretch them to fill a gap made by one of my surfacegenerated dips.. and theat it stretching down quite so far was making it have too many complex collisions.. I just decided not to have a great big tree there ... They're all enormous  trees that stretch to the ceiling of the sky, and I know from past experience that making them detail will cut a chunk out of them. (see the original Surefall map for more details)

well anyway, I'm posting a new post for my next new problem!