You need to call Delta to register the pet in cabin. They only allow four pets per cabin, so they will check that first. They will ask you for the breed, weight of pet and kennel (not to exceed 20 pounds, I think is the maximum) and the dimensions of the kennel. Different seats on different aircraft have different dimensions (window vs aisle, vs economy and first) and you cannot sit in cabins that have "lie flat seats". You also cannot sit in a bulkhead for exit row seat. I travel with a dog quite often and no one has ever asked to weigh the dog and kennel or checked the dimensions. I think they just "eyeball" it. Delta will inhibit online check in to make sure you check in and pay the pet fee (currently $125.00 each direction). The pet must stay in the kennel at all times. Of course, you could get an agent who decides to "go by the book", but I never have had that issue.