how many inches should i cut and make them?

Take out everything but the spring and put in the pvc pipe. Mark where the pvc sticks out of the forks and add to that how much preload you want. I think some add 1/8" or a little bit more. I was going to do this and add like 3/8". I'm not sure about this so I would get another opinion.

All you have to do is take the caps off and add spacers to whats already there and button it up.When I did mine I cut a set at 15, 20, and 25MM. then try them out and youll find that you dont need very much for most riding, just keep in mind that this is not for big junps just for better handleing. The dr650 jumps like a concrete block with wheels anyway:ride: (What I would give to have some rebound dampning on the rear)

Is that 1.5" pvc that folks are using?

1 inch i think

I turned down aluminum pipe (overkill yes, but if you had a lathe and the pipe what would you do?)
I just measured the OD of the leftover and it was 31.4mm.
For height I went with 15mm, as stated make a few and try them. 15mm gave me about 2" of sag