Anything but Huge and Tiny. The largest I go is Pointer/Gordon Setter size(about 26"). And even then, I do not like overly large dogs that are not agile. My dogs must be quick and agile. The smallest I go is Jack Russel Size.
I like the bigger dogs but also like Corgis and there were a couple of Papillons at the last agility trial I worked that I would have taken home in a heartbeat...