Of course you are confused. It has never been so confusing to size a bike. Look at those little thumbnail drawings. Seat tube length is what? And what if the mfr decides not to put the seatpost clamp roughly where it might be if this were a (non-existent) level top tube bike. Going to the chart for the Roubaix we see that seat tube length on the 58 is 50.5. No, you cannot make sense out of that. Standover height is where on that sloping top tube? And what if on a quick dismount you land a bit further forward?
Seatpost flex is now a big part of ride comfort and bike handling. Who has the formula for how to size the frame for optimum seatpost flex?
Best is to have the bike in front of you. Or underneath you.
Going way backwards to classic sizing you would be anywhere from 57 to 60cm on frame size. You could ride any of those, they would not feel the same. Kind of useless because current era designers do not even remember classic frames. Although they make nods on that direction when telling you the 50.5 is a 58. For new stuff S, M, L, XL makes more sense. The Eddy fit and French fit charts are purely a joke. Eddy did not ride bikes like that. Old Frenchmen did not ride bikes like that. Who gets what information from any of this?
Try hard to get some test rides. Don’t spend real money on a retail bike if the fit process is less than clear.