actually, in my prev post, i was referring to the waveguide-style
cantennas such as the one on this page:
http://www.geocities.com/lincomatic/homebrewant.html
i haven't actually built one of the circular "pringles-style" yagis,
but my understanding is that the can is not a waveguide, but rather only gives mechanical support. therefore, you wouldn't even need to use metal, nor would the diameter of the can matter. what does matter in this kind of design is the dimensions of the metal cup which the probe sits in, as well as the distance of the probe from the back of the cup. from what i can see in this photo, the design is wrong because the probe is just attached to the plastic and has no electrical connection to the metal reflector. of course, it's hard to tell due to the angle of the photo. in a proper design, the probe should be attached to the wall of a short metal cylinder which the back end of the yagi sits in.