I use a 105mm - it was chosen for my 6x7 negs (I don't shoot 6x6) but works perfectly well with my 6x9.

My Schneider was a smaller thread actually (older model) so I had to buy a step-up ring for it to fit my 39mm lensboard; just picked up a Rodagon 150mm, which has a 39mm thread. If I remember correctly, LPL make different sized lens boards, so even if you picked up a different thread size, you could still fit it to the enlarger.

The 105mm I picked up was much cheaper than many I looked at (possibly because of the thread size??), so even with needing to buy the adapter, it still came in under what most of the 39mm were going for. It's an older model (60s/70s maybe) but it's pin sharp, has great contrast, and in my opinion a real bargain.