aero bar elbow rests width
I've got mine pretty close together to hold my aero drink bottle - haven't had any issues w/ problems breathing. From my perspective, I would gauge it on comfort more than anything - especially if you're going to be doing long rides
I'll agree with it coming down to comfort. I'm still doing some small tweaking with mine, but right now the pads are a bit inside shoulder width apart. The extensions are closer together than that, so that I can sort of hold my own hand/interlace my fingers while I ride.
I use the aero drink bracket to get my aero bottle to fit on that setup.
You dont want them shoulder width apart. Having them more inboard will not restrict breathing if you are set up properly. With a decent amount of drop, and arms at 90 degrees, you should not feel much compression at all in your chest.
For an experiment, you you arms out in front of you like you are getting reay to do a rep on a butterfly machine at the gym. Then bring your arms in and touch the forearms together. Not too bad, right? Do it again, this time with your elbows lower, say around mid chest, to simulate a position without drop. Bring your arms in. Theres a bit of a difference between the two.
Mine are inline with my knees.
Life is short. Play hard and get dirty doing it.
Mine are also about inline with my knees, it looks like. I always just picture it in terms of shoulder width since that is the part I was told mattered for aero reasons.

How do you determine the spacing of your aero bar elbow pads? Right now they're pretty close together and I've been told this is aerodynamic but others have said that you want to space the pads apart so you don't constrict your chest for breathing.