Got some backlit signs at my stepfather's car dealership to fix. Anyway to know what kind of lamps we need before we rent a lift?

I see lots of T12 in older signs, usually 4 or 8 foot.

Most fluorescent signs in North America would use either HO or VHO lamps, usually T12. As for the size, it depends on the size of the sign. The lamps can be 18 inches, 2, 3, 4, 5, 6, 8, 10 and maybe 12 foot (or any combination thereof), depending on the complexity of the geometrical shape. The best bet would be renting the lift for the day, and get up there early in the morning. Don't do this on a holiday or a day on which the electrical supply stores are closed (some are closed on weekends). Then you can go to the store and get the lamps you need upon discovering what's in there.

Another solution would be to hire sign lamp replacement services from a company that specializes in that. They would know how to get the proper lamps quickly and do the work for you.

If some lamps have been EOL for some time, you could have ballast issues as well so all new lamps might not get everything lit up.

My stepfather's cheap so hiring an electrical contractor to just get the job done isn't an option.
He asked me if we should replace all the ballasts while we were at it. I suggested that's something you don't do unless they're actually bad.

How long will you have access to the lift ? You could first replace all the lamps and then replace the failed ballasts as then you would know by the unlit new lamps.

Can you get pictures of them?
The most common sizes are
-4 foot HO
-8 foot HO or VHO
-3 foot HO
-2 foot HO

It should be easy to tell what size lamps it uses since the lamps are usually vertical. How long will you be able to use the lift? If it is for around 3-4 days you can look inside and see what kind of tubes are in it and then order new tubes from elightbulbs. They are located quite close and new lamps should arrive in around 2 days.