I have the option to purchase an Aethos expert di2 2022. I’m 173cm, 141 lbs. Unfortunately, there’s no real solid option for test riding anything because of the bike shortage. My gut is to go with a 52 BUT specialized recommends a 54 (my size medium TCR advanced is roughly a 54 and has always felt a little large for me). Any input from those how have been in my shoes would be appreciated!
The best starting point is to find your current bike (year and model to get the right starting geometry of what you have and know), and compare using tools like the following:
Unfortunately, knowing your height is only a small part of the sizing equation. Legs vs torso vary between riders of equal heights, and can lead to a different frame size being appropriate. Considering you have a gut feel on your current bike, take that and check against the optional sizes to inform your decision.
I’ve been trying to understand Specialized sizing recommendations. I’d ride a 56 but they recommend a 58. If I built a 56 it would have an 11 or 12 cm stem. Specialized ships their 56/58 with a 10cm stem. I’ve been wondering if this is a design/fit philosophy on their part.
I could make the contact points exactly the same on both a 56 or 58 so they both could fit. It kind of comes down to whether I’d want a slightly longer wheelbase.
I would do what Chad says and check the stack and reach of your current frame and then from there calculate what you’d think would be the perfect stack/reach. Compare both of those to the Aethos geometry.
Just get the same size as you would for the Tarmac…identical geometry. Problem solved.
Like Chad said, your height is just one piece of the puzzle. We need more details before anyone can make a recommendation. Also second his recommendation for using geometry comparison sites.