I'll echo the sentiment that a proper fit for a drysuit is essential. Living in a cold water environment (Prince Edward Island), the majority of my diving has been in drysuits.
Ideally, you should need to put very little air into the suit to prevent suit squeeze. However, the suit SHOULD be somewhat large, to allow room for thermal underwear. I know that my own suit can feel a bit bulky during the borderline time of year when it's almost, but not quite, wetsuit weather, and all I need to wear underneath is a very light set of longjohns. But when the water is around the freezing mark, I need the extra room for the big woolies and padding for inside the boots. I definitely recommend a custom fit suit, but make sure when you are getting your measurements done you keep in mind what you'll be needing to wear under the suit as well.
And, I know this is going to get me a well-deserved scolding by a few of you, but I never wear a BCD with my drysuit. I know, the suit shouldn't be used for buoyancy, but I find it bulky enough without adding a vest over the top. I tend to dive with as little weight as possible, and in fact sometimes have virtually no air in the suit, preferring to live with a bit of squeeze. At the surface, even after dumping the air, keeping at eye level isn't a problem. It's really a comfort thing, and since a lot of the dives I do are working dives around aquaculture leases, I like to wear as little as possible that could get tangled or caught on lines.
Personally, while I would never trade in my dry suit, I much prefer a wet suit when it comes to comfort and mobility, not to mention the added bonus of being able to dump 10-16 pounds of lead off my belt. As soon as the water is warm enough to bear it, the drysuit goes on the rack.