The International Cessna^® 170 Association

Could someone tell me the correct size of rivets for the rear window?

I'm changing a windshield right now on my XP....it's a lot of work and stressful. Once you've changed a few, you realize how much work it takes to make it fit right, much more work than making it fit into the retainers.
You won't like this answer but; the one that fits the hole, would be the general answer if the rivet just picks up the window retainer then it's likely just a 3/32 rivet otherwise it will be a 1/8. After that ( as windows in older planes may get changed more than once) if the hole got bigger, a number 6 screw would be used which would do the job especially if the window needs to be changed again but on the downside adds weight.

We used #6 screws one one window. It tends to interfere with the upholstery but it did simplify installation and looks nice from the outside
I'll probably use #6 screws and half height nuts and a thin washer next time.

Just to close the loop here. I ended up using some AN470AD3-3 and a few -4's. If you are careful drilling out the old rivets with a #40 drill you will not enlarge the hole and an AN470AD3 rivet can be used. If the hole ends up needing to me enlarged for some reason you can use the NAS1242AD that have a smaller manufactured head but an oversized (.125) diameter shaft. This would preserve the appearance from the outside of the plane. You would only tell the difference by looking at the shop head once the rivet is set.

Just had to remove the paint at each rivet. Not a huge deal for me as I hope to get the plane painted soon. Anyway new windows are in and I am happy.

New door seals?

Lol. Yeah. New everything. Window, seals and door seals from Brown Aviation.

I think you can use "A" rivets which would unlikely stretch things when you put it back together. A bit late now but the A rivets would do a nicer job.

Jim,
I think soft rivets would be fine for this application as well.