I'm looking at getting a new road bike, specifically a Domane, and was recently fitted in order to narrow down the size. My fitter recommends that I get a 54, specifically because I need the steeper STA to get my fore/aft in an acceptable position. He also said the 54 will allow me to lower my bars some in the future. On the 56, my stem would basically be slammed if I set the bars to the same height as on my current bike.

I've read over and over again to size the bike by the top tube length, but the reach difference is only 3 mm between the 2 sizes, and STA is only .4 degrees. From a fitting perspective, am I correct in thinking that main difference between the two is stack height, and how high or low my bars can be fitted? I want to make sure I don't get it wrong in regards to reach if I get the 54.

The fitter may have over simplified his/her answer to keep from getting into too much detail that may confuse many people that just don't know or care.

Give them the benefit of the doubt and ask them to clarify their answer. Mention what you said here and let them address that.

As for seat tube angle alone, a 1 degree change on an arc of 60cm is not very much. About 1 cm I think. But top tube and other things might have changed on that particular model between those size frames.

Be careful, Reach is not independent of Stack. Latter affects how former is measured (similar to how STA affects ETT). This issue comes up regularly in regards to the Domane b/c of the big jumps in stack height between its sizes (due to longer head tube), making the reach differences look small on paper. Reminds me of this horrible thread: Long legs, short torso - Go go gadget legs

Your correct. There is only a 3mm difference in Reach with the rest of the difference being behind the BB. The main difference is Stack (effectively handlebar height).

Thanks for the replies. I did test ride both sizes, and I have to admit the reach felt very similar, but the 54 had a shorter stem making it feel ever so slightly cramped. I think I liked the way the 56 felt better overall, but I don't want to discount my fitter's advice. We had to cram the saddle all the way forward on my current bike to get the fore/aft correct, and he said I'll benefit from the steeper STA on the 54 compared to my current bike's 73 degree STA.

This would make sense if you have shorter than average femurs (your thighs are proportionally short or you shins are proportionally long). Does this describe your legs?

I'm not sure about femur length, but overall my legs are on the shorter side. In the past, I usually liked 56 top tubes, and ran a lower saddle, but it was kind of an eye opener that I needed so little setback. On my current bike with a 55 top tube, I've gradually moved my stem up from 100 to 120, and even that feels slightly short when I'm in good cycling shape. The LBS suggested I come back and take some longer test rides on both bikes, so that will be my next step. Thanks again!