I just built a 50cm Mark V. I'm 5'6" with a 29" inseam. I am running a 100mm stem and 167.5 crank arms. I might go to 170's though to get my seat a bit lower. Otherwise the bike fits nice. You'll probably will be fine with the 50cm.
Do you plan on buying it built or buying a frame and parts. If you plan on buying it complete you might ask Tony at IRO to leave your headtube long so you have have it sized and cut once you know how you want it.
