I am checking out preformed ponds (so I can take it when I move) and the largest I can find is 270 gallons. This seems like an acceptable size for a temporary pond until I buy my dream home  . But the distributer (MacCourt) suggests you only keep 6 3" fish in this size pond. Why? Are stocking levels so different in ponds? Can those same 6 fish grow to there full 12" in this size pond?

You can disregard the seller's recommendation of 6 3" fish for that size pond, because one would need to know the exact type of fish. For example a koi needs around 300-500 gallon per fish at a minimum , so six 3 inch koi would not fit in the 270 gallon pond after about a year.

The stocking level really depends on the filtration system and water turnover rate. Most ponds have a much  lower water turnover rate than what one would have in a typical aquarium, so the stocking level is much less.  So the proformed pond you're thinking of getting has a 300-500 gph pump , which has turnover rate of 1-2 times an hour vs. water turnover rate of 10 times per hour in some goldfish tank. It's often not practical to have such high water turnover rate in very large ponds due to the expense and space requirement of such a system and the cost to maintain it, so one has to reduce the fish stocking level.