"Camera lenses no larger than 20cm are allowed"

Has anybody gotten any clarification about this rule? Does 20cm refer to focal length or physical size? And if it's the latter, does that refer to its retracted size or full size? I have a 70-300mm SLR that I'm going to bring with me and am hoping to be able to bring it into the venues. I took a similar one to Atlanta in 1996.