Not really a max size for a cooler, regardless of element size, you don't want it touching the sides, or heating the water above say 180 - 185. I will admit I have had my HLT boiling while being absent minded during a brew session in a 48qt Ice Cube w/ no apparent ill affects.
The cooler is only "withstanding" the heat of the water, the element size is somewhat irrelevant, unless of course you fall asleep at 100% full on.
Unless you are looking for batches above ten gallons, I would think even a 4500w would heat strike water fairly quickly, and would heat plenty of sparge water during the rest. But if you got the juice avail., no harm running a bigger element.
Just a heads up, give the tun a quick stir while checking temperature, it can stratify top to bottome while heating.