Honestly- I don't know what your experience with women in real life is, but I'm astonished how much lower my size preference is in person than online.
In porn I'm regularly watching women over 300 pounds, but my actual wife is 225, and I find her body to be easily big enough as is. She sat her ass on my chest the other day, and I genuinely couldn't breathe. When she's riding me I can't reach all the way around her back; I couldn't reach her buttcrack if I wanted to, and I'm 5'11" with long arms.
Basically, if you're interested actually dating fat girls, give it a try with someone in the normal 200 pound range first- I think you'd be surprised how big they actually are.