Compare stack and reach. That is how a bike should be sized for you. If you can't find stack and reach for the two bikes you want to compare, use horizontal (effective) top tube length and head tube length. That works pretty well too. More simply, many bike makers use M for a size 54 and each size on bith sides of that differs by two cm. That is not universal however. But even if Ridley used numerical sizes, you would want to confirm the similarity to your old frame of the stack and reach or top tube and head tube.