There's no way to tell, because the geometries will be different, including the drop from the dropouts to the bottom bracket. You just have to forget about your current 26 in wheeled bike and start over - because nothing carries over. But I can tell you right now that if you use the standard road bike sizing formula such as available on Colorado Cyclist, this will give you a level top tube that you just clear enough (that's the inseam length times .65 or something). Then you have to figure out how much reach you need and find a frame that gives you that without having to use an extremely long or short stem for the size of the frame.