FIRST, by way of comparison (Everyone should include this when writing about product sizing!!!): I ordered two sizes in Regular (NOT Tall): medium and also small. My measurements are 73" tall, 40" chest, 15.5 neck, 34" sleeve length, 32" waist, 165lbs.
And I intend to wear this as an undershirt.
Size large, even after washing and drying, still fit me a little loose, so I ordered a small, which I like better. The small fits me well, although the shoulders could be wider, still it's good enough for an undershirt (If I were to be active while wearing it, small would be too tight.)--but I wouldn't want it any tighter, so I'll be washing it in cold water and line drying it. Quality is what you would expect for the price--good enough.
I'll be ordering some more in different colors.