Sometimes you have to use your imagination when it comes to "that's what she said" jokes. Anything that could be said in bed while having sex could be made into a "that's what she said" joke.
It's important that the pronoun "it" is used. That way, whatever the antecedent (topic) is, it can be replaced with a sexual reference.
It's so big.
That's what she said (about my penis.)
This is hard.
That's what she said (about my penis.)
I like it fast.
That's what she said (about sex.)