For me, it's less about the size and more about the mentality. Whether a person is class 3 morbidly obese or underweight, if they feel sexy and get turned on by fat, weight gain, and/or getting someone else fat - then they are the perfect size in my book.
I've dated all different sized people - including someone who was severely underweight, but was completely sexually confident with their size and were WAY into weight gain. I truly felt like that person was perfectly-sized. Another I dated was beautifully fat with a gorgeous fat belly, love handles, squeezable fat arms and even a cute double-chin just beginning to make an appearance. But they HATED it, and detested themselves, and no amount of positive reinforcement from me would change their mind, so.... NOT perfectly-sized.