Can you identify the size of this plumbing vent stack so I can replace the collar?

Hey all - In laws have a small leak that seems to line up with the collar here. I am thinking 3-4" collar is the way to go, but if you know that I need a smaller size, that would be great. Ill replace both while I am up there. I can order these in advance rather than going up, measuring and then going to the store... House from 1950. Both are metal. Thanks

https://preview.redd.it/03d5yjwr636h1.png?width=866&format=png&auto=webp&s=cc6354775126d6b582e7f9d7c8fb2e32e74f46e2