He was way taller than Buffy at the end. Cissy doesn't seem to grow at all during the show.

I'll have to keep an eye out for that. I think I'm currently on the 3rd season. Kathy Garver was 20 years old when Family Affair premiered, so she might have been done growing by then.

Yes, I think it would be unusual for a girl to continue gaining in height once she'd reached her late-teen years. There was a girl in my 8th grade class (age 13 or 14) who was the same height as me -- our math teacher had us stand back-to-back. By the time we were seniors in HS (17 or 18) I was a lot taller than her.