What do the existing park rules state regrding the size of dogs allowed ?
If the rules state no large dogs you have two choices either change the dog rule or inforce it. If you are going to have rules you must inforce all of them or none of them.
If however you are planning on introducing a new “small dogs only rule” I would include a grandfather clause for existing dogs and inforce the rule for all new applicants.
I have found that potential tenants with large dogs generally do one of two things either withdraw their application and look elsewhere or lie about the size of their dog and try to sneek it into the park. The ones that sneek in oversized dogs get an eviction notice.
Based on this I would guess most of your tenants with large dogs will likley leave. If you do not mind if they leave then inforce the rule.