Under Armor is pretty tight. It seems to fit me a half size smaller then what it says, so if medium is tight on you now for regular shirts its probably gonna take a large. Go out to Target some time, they have a line called "Merona" in the PJ/mens underwear/socks section. I have found, after wearing it under a vest in both a frigged winter and a hot summer, it to be far superior to underarmor in 4 ways.
1. Its available in a lot more colors then Underarmor
2. Its softer and doesnt have that as abbrasive a rub as underarmor *(important when wearing a vest)
3. Its cooler when its hot and warmer when its cold
4. Its about half the price
Deffinately try it on though, I thought hmmm... undershirts I typically wear a small. Yeah I couldnt breath in a small, medium is perfect size on me and Im a thin guy.