Want to put some road tyres on 18mm wide 29er rims to do some fast touring.

Whats a good size to look for?

Also

What minimum size road tyres can i get away with (I do have some 23mm gatorskins already but i think that might be pushing it)?

Why go narrow?

Wider is more comfortable and no slower (bar weight).

What width you go for depends on the roads and the weight you will carry. 23 ok for minimal kit and decent roads.

Narrowest I could go on some on one 29er rims was 28mm – tried some 25's but that looked like gravel rash waiting to happen. Just used some cheapish Conti's when building a commuter for a mate and they are still going strong a year later after a daily 8 mile each way commute.

sheldon says 28 here.

Wider is more comfortable and no slower

just changed from 25s back to 35s and it feels like i'm cranking up a car tyre!!

For the pedant, I should have prefixed my post with "all other things being equal" 😉