The Exit MkII is a great fit bike, it's the one I use - It gives me confidence to hear that the proper tooling was utilized. There's a lot of logic behind using a crank (170mm) that will come on the bike. Bicycle manufacturers (and component manufacturers for that matter) are a bit behind the curve in learning about the value of short cranks. Innovators like John Cobb (Speed and Comfort) are quicker at following science, trends and can produce short cranks far faster than the big boys (Shiman, SRAM, Campy) and bike companies have to catch up spec'ing their builds with the stuff we need. I'm confident you'd be better off with a crank shorter than a 170 but who wants to buy a brand new bike, pull the cranks that came on it and buy shorter cranks for Speedandcomfort.com or Rotor and install. It's a bummer but for those chasing perfection it's a must. The cranks are one part of this story... the saddle and where you perch is another.
There are triathlon specific saddles: ISM Adamo, again Jcobb at SpeedAndComfort, and others that make saddles specifically for triathlon. And then you have to find the model that's comfy for you - and there's no other way to do it than to get in your position and have the saddle put under you. The Canyon Speedmax you order will very likely come with a Selle Italia Watt Superflow - you might perch out on that and love it. If you don't start trying other saddles.
Shorter cranks would be nice. The proper saddle is MANDATORY. Now, here's where it'll effect your fit. When you move to the nose of the saddle you're going to be more powerful.. To make that position work you're gonna want your front end (read: aerobars) to move forward for sure and probably down a bit. So, back to the prescription: it's still a Speedmax size Medium with the stock 80mm stem....but now the arm cups might be in the middle hole (you can adjust this yourself in 90seconds: velcro pads peel off, 4mm hex bolts come out and move to a new hole). And, maybe the aerobar pedestal might be 45 or 50mm instead of 55 (you can solve this yourself in ~3min: 5mm bolts come out, different size spacers go in with appropriate bolts to match - it's all in the box).
You should order the Medium with confidence. When it arrives put it in the position the fitter created and start riding. You'll likely make some tweaks after each of the first few rides: cockpit forward, cockpit down, aero extensions tilted up and grips turned in a hair, saddle nose down 1 degree, etc...... and long term changes: 160mm cranks, sexy wheels, some BTA (between the aerobars) hydration system, etc.
My mom came from super humble beginnings and she's done well. In her life she's owned 4 homes. She once told me "your never finished improving you house 'til the day you sell it". I feel that way about bicycles.
Following up to let you know I got the Medium Speedmax CF8 put together and ready to rock. I am taking it up to Pedro Dungo in San Fran on Thursday for a final fitting. Will get some pics/final thoughts posted by this weekend to see how the prescription of medium ends up!
Thanks again for your time