This article is from the Model Rockets FAQ, by Wolfram von Kiparski with numerous contributions by others.
Again, as far as we can tell, there are no rules which define maximum sizes,
weights, total impulses etc. Most model rockets that can be bought from model
shops will take a maximum of 3 D-Class Estes motors, either clustered or
multistaged. There appears not to be an equivalent of an FAA Waiver for
launching rockets greater than a defined maximum. However, all of the HPR
enthusiasts I know do contact the CAA (Civil Aviation Authority) if they
wish to launch over approximately 3000ft.