I have a bit of experience that bears directly on this topic.

A while back I made a bunch of linear guides for a control stick
mechanism that used 5/8" steel push-pull tubes. I tried a bunch of
materials before settling on Nylon.

Anyhow, I found that in practice the smallest reasonable clearance hole
for the 5/8" tube was 1/32" oversize - that's 0.656" decimal or 21/32"
fractional. Any smaller, and the combination of crush and misalignment
used up all of the margin and the tube would bind up in the guide. The
1/32" clearance didn't seem to add any appreciable slop to the system,
and ran with reasoanbly low friction.

Another thing to consider is the that a lot of these low-friction
plastics have pretty steep thermal expansion curves. Also, some of them
absorb water and swell up.

If you have relatively low radial loads and you want really, really low
friction, you can go with these low-cost 5/8" ID linear ball bearings
from ACS:

http://www.aircraftspruce.com/catalo...shrodguide.php

I use a lot of them (copies of them, actually), and they're really
sweet. Even with a 12-foot aluminum tube run through five of them, the
friction is so low that the tube slides downhill under its own weight
at angles as low as 2 degrees.

Thanks, Bob K.
http://www.hpaircraft.com/hp-24