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.)

It's not a joke in itself, but the phrase makes us imagine there's a double-meaning in the last thing said. Another (older) version is "...said the actress to the bishop".