COG = Center of Gravity
Stack & Reach =
Stack & Reach Primer: Chapter One - Slowtwitch.com
To know the difference between a 56 and a 55.5 you need to know a bunch of different measurements. Stack and Reach can be either measured, if you physically have the bike, or calculated. If you have your Six13 then follow the diagram posted in the Slowtwitch article to measure Stack & Reach.
In a nutshell, there are 3 contact points on a bike; feet, hands, arse. They can be moved on a bike, but you are who you are, and those dimensions are fixed by physiology. You know where the BB is on a bike, then you know how high the saddle has to be then you also know where your hands will fall, there is no changing that. Now if the Six13 fits well any bike frame with the same or similar Stack and Reach will fit when fitted with the same components. If the reach is longer, a correspondingly shorter stem will put you in exactly the same position. Physical human size vs bike sizing, the human always wins. Same with Stack (for the most part, some anal retentive souls will argue that head tube angle screws this all up, the issue is marginal at best), if the stack of the new bike is 5 mm shorter one more 5 mm spacer will fix it and visa-versa.
This sizing question comes up every six months or so on BF, the answer is always easy: Stack & Reach. But
easy is not the BF way, we need a few pages of half truths and junk theory before you get any useful information.