Trek sizing

Your height is only one component of your size in terms of which size might be the best for you. Arm length, leg length, torso length, etc - all of these matter in determining a decent fit.

The guy might be right - or he might be wrong. It's impossible to say on the information you've provided here...

Not the same bike (obvs), but I ride a 54 medium Trek Emonda SL6 (H2 fit, 2019) and its perfect size for me. A 56cm would be miles too big for me in most brands I think.

For reference I'm about 5ft 10in so similar height. It'll depend on how you're made as well. I have proportionally shorter legs and longer torso. I also ride two Scotts, a 52S Foil and a 54M Addict RC. I only need a 10cm longer stem (120mm) on the Foil, as although the effective top tube is quite a bit shorter than the Addict, the reach is actually less of a difference due to the different frame tube angles.

Another thing to consider is how much saddle to bar drop you have, as this may also dictate your frame size. A bigger frame would have a taller head tube, meaning you might not be able to get the bars as low as you'd like. Similarly one that has too low a head tube could mean a huge stack of spacers under the stem.

I'd like to know what the shop guy is basing this on, as there are a few variables. The last thing you want to do is buy a bike that's too big, whereas its quite easy to make a smaller one fit you properly. Being cynical it sounds like he's maybe just trying to sell the bike he has in stock.

A good idea is to take the stack and reach measurements from frames you know fit. This is a great tool for comparing, as it not only gives raw numbers, but also the differences between sizes (and brands) -
https://geometrygeeks.bike/

Hope this helps

First, need to measure the bike’s frame size to make sure it suits your body type. For instance, if you want a Trek 18.5 frame size, you should get one with an ML Alpha size, which is recommended for people with a height of 5’6.9” to 5’10.5"