Ask them to describe what kind of 'build' they are. When you ask that, it is basically asking how someone is made, or literally built.

I think you should ask him the following question: "What do you look like?"

Shape would be understood, but it wouldn't sound quite right. In many English speaking countries asking about body type can be offensive. Online it probably isn't as much of an issue. I would say it: "What is your body type?" Leigh's answer about build is good too, and Vandor's would be the most common way you would hear this question asked.

Please describe your appearance.