I think you've told one like this before.

soroban wrote:

The midget fortune-teller who escaped from prison
        is a small medium at large.

I've seen this sign outside a police station.

The police station's toilets mysteriously disappeared.
The police have nothing to go on.

I think this one's the best.

The fattest knight at King Arthur's round table
was Sir Cumference.
He acquired his size from too much pi.

They're all very funny, good job, soroban!