Because of network effect, I'd probably advise against any kind of niche or specialized dating web site. Especially if there's a monthly subscription to send and receive messages, and there's no other value proposition.
en.wikipedia.org/wiki/Network_effectThere's plenty of fat people on general purpose dating web sites.
But as always, with online dating web sites in general, keep your expectations low, and have reasonable standards. I'm skeptical of the fact that dating web sites are one of the few business models where satisfied, happy customers stop buying.
Also, keep an open mind. For all you know, you might meet your next significant other at a baking class, in the library, at the bar or night club, or even at the grocery store. Or potentially through friends. Maybe you work in an office building, and over lunch you see someone who works at another company in the building. Or after a concert. Point is, you never know!