Don't look at me like that. The huge joke HAD to be made! HAD TO!
Susan is five feet tall in this and the previous comic, making her pretty much eye level with Rhoda. Now, when I say that, I'm not counting hair volume, and I never do. My characters have pretty fluffy hair, so that can add quite a bit.
Incidentally, I am also not sure how Tedd managed to do as she describes in panel one. Epic fail, Tedd.