Get a LeJeune (531 double butted throughout) or a Gitane, same with the tubing.

French to French can still catch you with seat post diameter, usually between 26.2 to 26.6 with 26.4 pretty common. Headset cup diameters can differ too, if one is going from a Stronglight Competition to Campagnolo, reaming the frame and cutting the crown race seat are needed.

steerer threads 25 x1, quill diameter of the stem 22.0 mm.

BB threads 35 x 1 68mm wide, very typical until the later 80's.

My first guess would be a 56cm frame center to top, as measured back in the day, no bigger than a 57, no smaller than a 54.

https://sheldonbrown.com/velos.html
https://sheldonbrown.com/frame-sizing.html

Should be able to find all the answers at these link. Good luck.

I am 5'8" tall and buy trousers with a 30" inseam. My ideal road frame size is 55cm C-T.

The Reynolds 531 70s Jeunet that I have (back-burner project) does not take a 26.6 mm seatpost, that's for sure. Right now, I don't have any smaller ones to try. Post opening measures 25.8 or 26.0 with a dial caliper; but it's also a tall frame -- 63 cm C/T, so perhaps it has an especially robust seat tube in terms of wall thickness. It's not a particularly heavy frame, so I do believe that it must be constructed of double-butted tubing, and it had Reynolds 531 fork decals that are "original," French, and worn nearly to nothing now; and a shadow where the R 531 decal would have been on the seat tube, in the right size and shape. I'm 5'-10, buy trousers with 32" inseam, and prefer bikes in the 60-61 cm size.