I'm all dick.
Means the guy is thin but has a meaty cock, like he's a twig with a sausage.
This guy is all dick. He's like a skinny kid with a monster in his pants.
He's all dick. He looks like he could run a marathon and still have time to jack off.
He's all dick. He's the guy who walks into a room and everyone's like 'what the hell is that?'