I am quite big. Often, I am the biggest guy in the room. I have never had an SP turn me down or even make me suspect that she is not attracted to me. I have had more than a few girls tell me that I am one of their favorite clients because I am clean, polite, reliable and funny. I have had more than a few girls go out of their way to accommodate me when they know that I'd like to see them. I have had more than a few girls provide me with services (at no extra cost) that they do not advertise and which they say they don't really do with punters (rimming, CIM, swallow, extra time etc).
Now, I realize you cannot really trust what an SP says, but I am not stupid and I can tell when someone is genuinely fond of me and when they aren't, but you can tell when you click with someone and I usually click with most girls, particularly the ones in their mid-twenties on wards.
As for positions, I try to avoid positions where all my weight would be pushing down on the girl. I stick to standing missionary (is that what it's called?) where I stand on the floor and the girl lies on the bed, with her arse hanging off the side. I find that it allows me to thrust away as hard/slow and deep/shallow as i like, without my weight being an issue. Also, doggy is great, particularly if I stand whilst the girl bends over on the bed. No issues there. Cowgirl is also always fine.
I would add that I was also on SA and I ended up sleeping with every girl I met, without any money being exchanged on some occasions, so if you are fat or heavy or a chunky bastard, you can make up for it with a little charm and personality....oh, and money. That always helps. You should try to be more secure about your size - women can sense insecurity and confidence is always a huge turn on.