I'm 5'10" weighing 167 lbs. My waist is 34" with 38" chest and i typically wear size medium slim fit tees.

I ordered extra-large per the instructions and reviews and the shirts hung off me like a dress. I re-ordered large thinking these would fit better, but they did only marginally so. I finally ordered medium and they fit exactly like an slim-fit undershirt should: hugging the body but plenty of room to move and plenty shirt tail to tuck in.

Maybe they've fixed the sizing issue. But remember, these are slim-fit undershirts. They're supposed to comfortably hug the the body. If you're more of a pear shape, maybe you'll need one size up.