Stack measurement help I'm trying to determine where I fit on the Cannondale Slate geometry because it doesn't seem they use top-tube actual or seat tube, but rather they're using the Stack? I see a great sale on a 56cm bike but that is classified as a Medium and I'm generally a large. I'm 5'11" with around a 33in inseam. Long arms. I believe my other road bike is a 56cm and I'm going to assume it was measured correctly for me. This is sort of an "all road" bike that will be taken on mild single-track, if that changes my measurement decision. What I don't understand is, it appears they're using stack as the bike measurement?

Is a 56cm too small for me? It appears the top tube and seat tube are tiny and has me really confused. I'll also mention I'd prefer not to ride with an overly aggressive seating position.

** edit - These are 650b wheels.
Stack and reach are a 'system' of alternative bike frame measurements always used together. They are supposed to be meaningful absolute values, unlike seat tube length (or 'effective' seat tune length, where top tubes are not horizontal). 

They are based on two datum points: the bottom bracket axle axis and the top of the head tube. Stack is the vertical distance between those two, reach is the horizontal distance.

If you have a meter long ruler (or a tape measure and something long and straight) and a level you can measure the stack and reach of your existing bike and make a comparison to the Slate geometry.