Angie:
Is the animal being classified as a "dangerous breed", or are they concerned just because it's bigger than their limit? As for the issue of liability insurance for a reasonable accommodation generally, you should read HUD v. Country Manor Apartments, HUDALJ 05-98-1649-8. It's in the case database at
http://www.fairhousing.com/index.cfm?method=case.view&caseid=2049 . It involved a housing provider forcing users of motorized wheelchairs to purchase liability insurance.
Also, I will be sending you an e-mail with a pointer to another resource you might find helpful.
Tracey