Right, start from scratch.
Not all buckets are the same, and if you're a bit chunky then you're going to struggle to fit into a lot of them. I don't mean superfat, just even a little podge can rule you out of the smaller ones: My Sparco Sprint V in the MX-5 is snug enough that when I had all my weight on I'd never have fit into it, and even at 5' 10" and 12st it's very snug. You need to have a good measure up, and even better you need to go and sit in a few. Demon Tweeks have plenty on display you can try out, so I'd suggest a road trip down there for the day as even if it costs you £50 in fuel it couls save you £1000 in buying the wrong seats. If you're anywhere near me you're welcome to try mine out for size.
Failing that, go and get them to send you one of their red Motorsport catalogues, they've got them all in there and with all the dimensions you could ever need. You can do the same on the website, but it's far more awkward.