soon2bff, I'm 6'6", 260# and I can tell you that finding an off-the-shelf suit of any kind is a hassle and a half. However, the fit of a wetsuit is more important than the fit of a drysuit (IMHO). I'm a broke college kid that thought spending the extra cash on a drysuit was stupid and/or wasteful....until I saw a drysuit diver. We were diving N.FL caves (68F water, 75F air temps). I was diving wet in a WORN 3mm j/j. I was warm enough during the dive, but when I got out of the water I was drenched to the core, frozen, sticky, had to change, etc. A buddy was diving dry. He was out of his drysuit with it packed and in comfortable, dry clothes ready for the ride home before I was even out of my wetsuit. Before I even started breaking down and packing gear, I had to go dry off and change into warm, dry boxers. He was sitting on the bed of the truck with his gear all packed up and ready to go in the bed of the truck, waiting to help other people get their gear together. This was followed by a 13 hour drive home. Guess who wasn't completely dry and guess who was completely comfy. I'll give you a hint: I now own a drysuit.
For how warm different thicknesses are, they ARE different. I've dove a 3mm warmer than any 5mm. It's about sealing the seams, fitment around the neck, wrists, and ankles, and the quality/age of the neoprene. As has been said before, any water moving in and out ruins a wetsuit. The way a wetsuit works is by sealing a pocket of water around you. Your body warms that water, which is insulated by the neoprene. Seams that leak make it harder for your body to keep the water around you warm as it's constantly taking in cold water and leaking your warm water. Bad fit around your neck, wrists, or ankles does the same thing. All neoprene compresses with depth, so the deeper you go the less good it does you. Cheaper neoprene compresses more and/or insulates less. Old neoprene is either pre-compressed, compresses more, and/or insulates less.
A drysuit (bilam/trilam, mostly) doesn't really keep you warm. It keeps you dry. The idea is a drysuit keeps you dry, the undergarments keep you warm. So, that's not completely true. Your air pocket still insulates you, and all drysuits have at least a little bit of insulation to them. Neoprene drysuits tend to naturally be warmer than laminate drysuits. A drysuit will wear, just like a wetsuit. But different from a wetsuit, as long as the drysuit is keeping you dry it's keeping you warm. Seals go bad, zippers go bad, inflators/deflators go bad....but generally the suits stay good (barring cuts/tears).
For trying one on: I don't know how all shops and/or how all suppliers work....but my LDS could order multiple sizes if you were between sizes and get you to try them all on before you purchased it. As for what I'd do: many drysuit manufacturers will do a semi-custom suit for little to nothing extra. Hollis offered a few buddies and me a semi-custom suit. Send in measurements, wait two weeks, get suit. They stitch together pieces of different sizes to get the best fit for your measurements, and modify some pieces if required.
Since I see you're looking at getting into PADI OW, it's clear you're new to diving. Welcome to [-]your new obsession[/-] [-]the darkside[/-] the club! One thing to note: Diving a drysuit isn't easy to do well. It's not hard to dive, just hard to get good and dive "pretty." I'd recommend renting a wetsuit and diving that while you learn to dive. I also recommend you gauge your interest and the longevity of your interest before investing in any substantial quantity of gear. I was addicted to diving for about 9 months and then didn't dive for 4 years. Now I'm diving as often as the weather (and cash) permits. Not buying gear then was a great decision. Buying gear now was also a great decision. Just make sure your purchases are timed correctly.
