Toothengineer,
As is so often the case with super bikes, there are overlaps in what bikes will fit and for you that's the case as well - but the Medium is the best option.

Option 1 - The Medium with the short stem that comes stock, the low spacer (bike comes delivered with the medium spacer [60mm], in the box is a high spacer [90mm], and a low spacer [20mm), the medium extension which also comes stock on this size. The draw back would be this - the seat post has mounting holes (can I 'em bosses, sure!) for a behind-the-seatpost-cage but for you, on the medium, those holes (well, prolly one of them) will be down in the frame and then if you want a bottle behind you it'll have to be a non-Canyon system that mounts on your saddle rails.

Option 2 - The Small with a long stem. low spacer, and the medium extension - the good here is that you get the seat post mount holes exposed and ready to use. There are two risks with the small: 1) is that you utilize the "customization" process during purchase and that will negate the 30 day return. 2) this is the longest the bike gets, you cannot go even 2mm longer in your cockpit.

Ian

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Ian Murray
http://www.TriathlonTrainingSeries.com
I like the pursuit of mastery
Twitter - @TriCoachIan