So I got the lens today and shot around with it a bit. It's definitely bigger than my Sigma 17-50, but it's much faster and on point with focus. Out of the two of them - if size is the deciding factor - I'd say go with the Sigma. It's got great image quality, and it's pretty small. I'm using the Canon for a wedding this weekend only because my Sigma is starting to misbehave after 5 years of service, but the Canon heavy enough that it's got me looking for my battery grip.

If you have to go with a prime lens to conserve space or make the camera smaller, I recommend going with something on the wide range, such as @pfunk's suggestion about the 24mm pancake lens. That thing is so thin it's almost not a lens at all, but more like a camera body cap.