yeah o-guage terrain is a bit big for most wargaming mini's but you can sorta fudge the scale. The mini's i come across regularly in hobby shops are usually what's referred to as 28mm 'heroic'. I also think it depends how they measure the fig...if it's from bottom of feet to top of head...or bottom of base to top of head.
If I were you I'd look around online for close out deals on Celtos mini's....I know Newwavegames.com used to carry them.
Also try Rackham.....or maybe Reaper. Hope that helps a bit.