Buying clothes online is usually very troublesome, there is no way to check the fit, and returns can be troublesome. First, clothing must be sold in US size. I have the XXL and it fits like a large, they should be snug on the chest and arms, flow in the middle and be a bit longer than regular t-shirts.
Ultimate Comfort: Discover the Comfort Colors Sleeve 6014 - X Large Men's Clothing, T-Shirts & Tanks
40 Review
Carhartt Workwear XXXXX-Large Men's T-Shirt β Original Clothing for T-Shirts & Tanks
34 Review
AM Fitted T Shirt 3 Pack for Men's Clothing - T-Shirts & Tanks
46 Review
Ultimate Comfort and Style: Minus33 Merino Wool Ticonderoga Lightweight Men's Clothing, T-Shirts & Tanks
43 Review