What size t shirt should I get?

[blindmelon]

hello I'm about to order 7 Ralph Lauren "custom fit" t shirts online. I don't know if I should get small or medium. I'm 15 I'm 5'7" weigh 130 and my 'raw' waist size is 29 but I wear 30/31 so it's not tight. The problem I've had with cotton t shirts is that the waist I think it's called of the shift always gets stretched, everything else will fit perfectly and the waist will be good for a few days but then it stretches out and deforms. I assume this is from spreading my legs when I sit down and the shirt is stretched out. The shirts aren't too long they usually seat right in the middle of the crotch zipper of my shorts. The Ralph Lauren waist sizes for shirts are 28-30 and medium is 31-34 I don't want the shirt bottom to stretch but I also don't want baggy looking shirts with 5 inch shorts. Any help thanks