you can get 1/2-3/4 yard out of a small mans shirts, but save time and go straight to the xxl sizes. 2+ yards per shirt! You don't need very many shirts to end up with plenty and more for a quilt, the problem is getting variety without ending up with a house full of someone else's used shirts.
I had so much trouble finding all cotton ones I picked up 3 poly/cotton ones, Eeuuu do not like that fabric to work with at all. Even starched it doesn't keep its shape well it felt chintzy to my hand.