Also trying to choose the right size for an IAx (the IA 14). My first tri bike. Got fitted to a stack of 559 and a reach of 433. Focussed on long distance for next year, so maybe for shorter distances and after getting used to this position I can get just a bit lower. Should I get a 56 or a 58? SuperDave, what are your thoughts on this?
Ha! I thought you were giving your pad stack and reach. I was certain you'd be a 48cm.
On the IAx you have the option of a very low pad position because of the clip-on nature of the aerobars. I'm still struggling to illustrate just how adjustable the front of this bike is. More than 100mm of vertical adjustment is possible from lowest to highest position.
The 58cm frame has less aerodynamic drag over a -20/0/20 sweep and the longer wheelbase aids in a stable, straight-line path.
I would recommend a "slammed" front end on a 58cm over a 56cm with 2cm of extra bar stack, 2cm more pad reach on the extensions with more of your rider weight over the front wheel.
It sounds like your inexperience with triathlon position might also better be served by the 58cm because it will allow for a upright posture as you acclimate to a "textbook" position.
In general on the IA, go as big as you can without going too
big as the slight aero hit of the smaller frame cannot offset an upright parachute position if you could have tolerated/adapted to a lower posture.