skinewmexico wrote an answer about Ridley X-Fire 20 SRAM Disc Complete Cyclocross Bike - 2015 on September 10, 2014
Just look at the effective top tube on a Ridley, and buy based on that. Ignore the fact that the size may not match whatever other bikes you have. When the frames were letter sizes, a small in a Ridley would usually be a medium in most other manufacturers. At least that has worked for me. Twice.