Terry makes some nice plus size jerseys that tend to fit loosery than regular jerseys. Also try mountain bike jerseys - they tend to be cut more casually and loose.

Also - many department stores now carry "fitness wear" these are basically tshirts made in various wicking materials (usually CoolMax). I bought some for spin class and like them so much also use them while riding.

I was talking with a friend last night about he old days when I only wore cotton tshirts. At the start of the ride they fit fine but the end, they were 4 sizes larger and went ot my knees due to all the sweat collected. It definitely pays to wear wicking material.