I'm 6-foot for 240 lb with a bit of fat and muscle what size should I get?

I'm 5'11" 235lbs, bought XXL and found they were a little loose and would defeat the purpose I bought then for. I swapped for XL and fits perfect.

Im 5'9 185lbs normally wear a large size tshirt, would a large work?

Large might be a little snug, you are my height but 20 pounds heavier, you might be better with an extra large. I also felt they didn't fit well in the neck and shoulder area, like maybe they felt it was on backwards. I bought Tesla, the fit was better and I went with a medium and a large, I have a 38 inch chest area and the large fit great and the shirt is a very good quality.

I’m a small guy at 5’5 115lbs is the Small best for work outs and under shirts?

My son is 5'11 and weighs about 145lbs. He wears them as shirts for basketball/running/hanging out and under his track uniform tank. The fit is loose but not baggy.

I'm 6.3 and 265 lb, would 3x be to big

I'm 5'11" 235lbs, bought XXL and found they were a little loose and would defeat the purpose I bought them for. I swapped for XL and fits perfect. If I was you I would get a XXL.