Right on for XXLs, then lost weight and ordered XLs, then more weight and zeroed in for Ls. At which point XXLs actually were still wearable, and XLs were fine, so were Ls. I realize this isn't you. If I just take a stab and say your 5'10-6'0" 155-175, GBW meds should be perfect.
I look for deals. bikenashbar was great last year. I subsequently ordered with worldcycling.com and competitivecyclist.com. At discount, GBW is a "steal". At MSRP, you won't be unhappy, performance-for-price. Assos makes really nice stuff, but it's hard to find great price-to-performance. I got an FI.Mille at chainreactioncycle for $196. A very good short. But Ozons at ca. $100 were better bang-for-buck. Xenon Sonics at $169 were waay better bang for buck than FI.13s.
If you're looking around, try something with "EIT" "Elastic Interface Technology", an Assos proprietary pad available in many non-Assos brands. It's smooth abrasion-minimizing and cushioning. GBW uses it in most of their shorts. I have DeMarchi and Craft tights with them. Good pads.