I went for 15mm, same as the stuff at Zapp. I got two 760mm rails and two 1150mm rails, with 10 bearing blocks all brand new from someone in Germany for 400 euros. Hope it arrives!

I will only need 8 blocks (don't think having 3 per rail will make much difference?), and I can only have the Y-axis 950mm long. That means I'll have two 200mm rails with two block going spare...cutting them could be entertaining.

What size did you go for?