Where will the size information be on a 1986 Peugeot bicycle?

You will have to measure it. Measure from the center of the bottom bracket (that's the spindle that the cranks attach to), all the way up the seat tube to the center of the top tube. That measurement, in centimeters, is the bike's size "center to center".

Some bikes (usually English or American made ones) are measured "center to top" in inches.

Take a look at this illustration:

On some 80's Peugeots you can find the size info typed on a small paper label affixed to the chainstay or to the underside of the bottom bracket shell (see the "63" typed on the tag on the blue example below). That info should match the "Center to Center" dimension that [MENTION=190364]lostarchitect[/MENTION] posted above.

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"Center to top" generally means center of bottom bracket to top of top tube, not seat post collar. If bottom bracket height is pretty average, it serves as a decent indicator of standover height.

"Center to center" is a better dimension for evaluating overall frame geometry, including angles and the lengths of the other tubes.

Today, maybe, but not originally. Seat tubes stick up more now than they used to.

Per Sheldon Brown:
"The old standard system was to measure from the center of the bottom bracket to the very top of the seat tube."

But yes, I agree with you, CTC is the better method (assuming the stop tube is level, which on new bikes it often isn't).

I always measure C to C. Not sure 8f it's a better standard, but I do remember that most manufacturer's catalogues list frame sizes that way.....
You have to remember that the height of the top tube from the ground can also be affected by the geometry of the frame. Some frames have BBs that are higher off the ground than others, which affects th height of everything above it accordingly. I assume that seat tube angle could also affect tube lengths ant height in some way. It's not an exact science, fitting a bike to a person. It changes a bit from frame to frame..
Case in point, I have a 51.5 cm Bottecchia that everyone, including myself, at first glance could swear should be a 53 or 54cm. But measuring the seat tube (C to C) something like 10 times already, it still measures 51.5cm everytime.....
Our forum's resident Bot expert explained that it is all due to the frames geometry.