Odd that they have such a specific body requirement.
When I think about it, their walkers are always tall and thin. I know that in the show, they like to use thin people with small heads, because they layer the prosthetics on top of them. If they start with someone slim/small headed, then it doesn't look ridiculous if they have layers on top of that.
For SDCC, a slim person probably better portrays an emaciated appearance, which is what you would expect from a walker. I think the 'tall' part is so that they can intimidate people. They always seem to want to 'lean over' me when I'm at the AMC booth/TWD Experience.