My barn cats will climb the side of my horse tank (about 3') and balance on the lip to get a drink
If your having a hard time finding something small, you can get a chicken water heater that is just a flat thing that the water bowl sits on. They sell them at most feed/supply stores and they run in the $30 range.
ETA- have you looked at bird bath heaters? That might work too. Either way you can use whatever size bowl you want.