Very Important Question (sarcasm)

 It's hard b/c they all use different brands of shirts to print on, some of the races I have been in the L was HUGE and even after I washed it in ho****er it was still too big, then the next one was perfect in a large and even a little snug after washing and drying. 

I say go with the large, and then google fun no sew ways to make t-shirts smaller (I have saved some great t-shirts this way!