I've had trouble in the past with long underwear shrinking in leg length while losing elasticity in the waist band, and having to frequently pull them back up to the waist after bending, etc.
I ordered the merino wool underwear, with the pants one size larger than I would normally have, and am pleased with the fit. Even if they later shrink, I feel that the leg length will still be good. I have a pair of merino wool socks and was so impressed with them in my boots that I decided to spring for the extra cost of marino wool long-johns. I've worn them once so far riding on a cold morning as a base layer and was not disappointed. (Posted on 11/15/11)