I don't know where you are, but before you spend $3000 on a suit and undergarments that might not quite fit consider trying to fit in a trip to a place that sells a lot of drysuits. Much as I like DRIS as an on-line shop, my suggestion is to get someone who sells a bunch of drysuits to measure you in person and order it from them. The mechanics of measuring need to be done very carefully and methodically. If you fit in a standard size they can then let you try on a suit and undergraments and ensure that you will have the range of motion you should because they know how to test what range of motion you have and need in the suit and undergarments.
BTW, if you want a slightly too small $2500 dry suit, I have one... And a $3500 properly fitted suit on order that I got set up on a trip to Florida. The slightly too small suit worked OK for me until I needed to reach the valves on doubles, which wasn't something I had thought I'd be doing when I ordered the suit from my LDS. And yes, they very carefully followed the measurement instructions. Neither I not they knew enough to realize the suit I got delivered 3 months later didn't have enough range of motion.
---------- Post added November 18th, 2015 at 11:33 PM ----------
So I was wondering if anyone can tell me how drysuits should fit. I have a 3mm Bare Velocity wetsuit in medium large tall which fits perfectly. I've been considering a drysuit for awhile now for colder water. My concern is with the thermal underlayers. Is it a good idea to buy a size up in drysuits to account for those additional layers? If it helps, I'm looking at both the Bare D6 drysuit and their Trilam Tech dry.
I have a trilam tech dry and have a few issues with that I feel like ranting about, perhaps this will help you.
The big issue with the trilam tech dry is that it doesn't stretch at all. For example, I can reach the valves on doubles wearing the base layer or a light fleece, but with the SB midlayer (which is great otherwise) I simply cannot touch the left valve at all. So sizing has to be exactly right. And their sizing guidelines don't seem to be really that good. My LDS did a very methodical job of measuring me and it still ended up too small.
The other big issue is that the exhaust valve is really poorly located. It should be on the outside of the upper arm, but is instead on the top of the upper arm. So when you are flat in the water the exhaust valve is below you, and the air bubble can't get out. So when ascending you need to have your fist against you ear so the expanding air in the suit can escape. Which makes it really damn hard to reach the dump valve on the bottom left of your BCD (or the lever on a Dimension BCD). Oh, and when you go to reach back for the valve on the left side the inch high suit exhaust valve hits your upper arm about 3 inches below your wrist and limits your range of motion.
The relatively minor issue is that Bare has no idea how to install things like pockets or p valves, so they fall off. Latex glue is not the glue you use for things that are supposed to be permanent. Have DRIS, Gamble Scuba or someone else competent install these after you have gotten it and tested it, don't get them factory installed. My suit has never leaked (umm - other then when I don't close the zipper...) but it's a standard size. The 3 guys I know who got custom sized Bare suits at the same time all had to send them back as they leaked at the seams.
