Their true forms are tall humans, about seven foot, and they are rather pale. Frankie and Benjy were revealed to be in league with the Vogons, because they want to stop everybody else finding out the Answer and want only the very rich to know. They tell Arthur they need to cut open his brain and thus kill him in order to know the Question, because he is human and survived Earth being blown up.
They came to Earth after it was made by their computer Deep Thought and they disguised themselves as mice, (which is actually possibly the closest humans could see their true forms) and they played a psychological game on humanity.
However, because they are mice at the time, they have limited power and agility, and as such are easily defeated by Arthur and his friends, and eventually come up with a new solution - to fool Management into thinking they have the Question when they don't.