You are heading for trouble like you have never seen. If the applicant meets all your requirements and you turn them away because of the service animal, they will most likely file a complaint. You will have to prove that they were disqualified due to other reasons ( not enough income, criminal background, etc). It appears that the only reason you would not accept this tenant is because of the large service animal.
I am wondering if this is a service animal or another "therapy" animal. Bona fied service animals are rare. Therapy animals are everywhere. Tenants use this ruse to sneak pets in the back door. Therapy animals have only came about in the last ten years. Before then they were just pets. Have you ever seen a therapy animal on an airplane? There are only pets on an airplane and they are in cargo. People who habor these therapy animals have more problems than an animal can solve. If you allow them, they will make some of those problems yours.