I would go into advance search and see if you can see the layouts that other members have made and posted photos. I have been pleased with how others have dealt with different sizes of t-shirts.
Examples:
http://www.quiltingboard.com/main-f1...t-t205021.html
scroll down and see linbails quilt
http://www.quiltingboard.com/main-f1...p-t255951.html
http://www.quiltingboard.com/picture...y-t255581.html
http://www.quiltingboard.com/picture...t-t257518.html