Some notes about a couple of the parks...
Dollywood is very forgiving of larger people. Tennessee Tornado has ample room, (I've never seen someone denied a ride on it) and Blazing Fury barely has restraints. Thunderhead's MF trains are a bit tight in the seat, but Dollywood has a seat belt extender! My wife and I, (we're both large people) can ride in the same row on Thunderhead due to the extender.
The two rides anyone will have trouble with at DW are Timber Tower and Mystery Mine. Both rides have test seats, so make sure to try them. Timber Tower has a formed plastic seat with a leg divider and over the shoulder restraints. Basically, if you can fit in a Top Spin, you should be able to ride TT without problems.
MM is unusual... only over the shoulder restraints, no lap belt. The restraints aren't ratcheters like most rides, but use hydraulics instead... there's no 'click' point to reach. (Where you pull the restraints down to is where they stay.) In order to ride, the green lights behind your seat must be lit. So, pull the restraint down to where you feel comfortable, and adjust from there.
BGE: I have a 54" chest, so I have to use big boy seats on every B&M that has them. I can barely fit into Alpengeist, but that was two years ago... if they shortened the belts this year, I doubt I can fit. Apollo's Chariot is usually not a problem... I've only been denied a ride once on it. Loch Ness is never a problem, although I do suggest if you're tall, ride in the front row!
Squid