I did the rack shown in the pictures.
It all depends on the thickness of the wood or material you are going to use for the base.
For the shown racks I used a 20 pound magnets for each pipe, and I'm not using bearings inside the pipes but a small cylindrical 6 pound magnet.
Maybe it is because my rack is angled, but it is the only way I felt the pipes were stable enough.
Maybe for a flat stand you can use lower rating magnets, but no a lot smaller if you are not planing to have some kind of groove etched on the base.
All the magnets linked by previous posters are way too small IMO.
They won't hold your larger pipes stable or vertical.
Here's a link to the thread and the pictures.
http://pipesmagazine.com/forums/topic/ultimate-minimalistic-pipe-rack
Cheers,
J