Amazon Essentials Men's Active Performance Tech T-Shirt (Available in Big & Tall), Pack of 2

Should i get an xs or small for my son who is 5'3 100lbs?

Looking to wear this as an undershirt. I'm 6-2 with broad shoulders.What would be the recommended size? Also do the sleeves on these shirts run long?

Can I iron vinyl decals on these shirts?

The size xl of this brand is like the XXL of baleaf. Can be ? somebody confirm that the Amazon Essentials Size Chart publication are correct?