Assuming someone is in-between sizes is there any advantage to having the larger or smaller of frame options for cyclocross racing? I know the trend in road is sizing down and adding a huge stem/seat post. Is one better for shouldering, bunny hopping, weight advantages, corner handling, not crashing, etc.?

My situation: Current road racer looking to do cyclocross. Will be racing a Crux, 6'1"with a 34" cycling inseam, currently on a 58cm Tarmac with a 120mm stem. This is by no means a "sizing down" situation. I recently picked up a 56 Crux from a friend and did a couple 25-50 mile rides on gravel/pavement... it feels smaller than the 58cm tarmac for sure but it worked. In fact, I'm thinking bunny hops and the like were probably easier than it would be on a 58. It almost made me second-guess riding a 58 Tarmac because it wasn't the earth-shattering difference I was expecting.

Sub-question: Would going from a 58 road to 56 cross bike be foolish for training? In other words, should I try to keep the bikes as closely fit as possible so I'm using the same position for each?

Anyway, I went down the rabbit hole of looking up pro bike setups for reference and I found a guy about my height and weight riding a Crux: Pro bike: Zdenek Stybar's Specialized S-Works CruX | Cyclingnews.com

This guy is using a 54cm Crux! To be fair I'm a couple cm taller than this guy but I was kind of amazed to see a 6' guy is on a 54. It looks like the cross bikes have a longer wheelbase but much shorter top tube, so it's kind of a bigger bike but with a more compact fit?

Now I know "everyone is different" and I "should get a fit" (probably will soon) but I'm just trying to educate myself so I'm not just taking one fitter/salesman opinion. Thanks!

TL;DR Is it better to go smaller or larger for a cyclocross frame?

Smaller is better, or to be more specific, a slightly shorter reach/ETT compared to your road sizing is common. Some companies already incorporate this into the geometry so check the charts to compare. Some cyclocross racers like to be fit the same as their road bikes, but the majority have a slightly more upright position, probably due to the rigors of racing and the strain it puts on your back etc. Also realize that the position should be optimized for the hoods where you will spend the majority of your time. If you want to match the fit to your road bike during training season that is much easier on a slightly smaller bike than a larger bike too.

You should be fine with how you are now. You spend a lot more time on the road bike so keep that comfortable for that. CX you are in and out of the saddle. on and off the bike etc you're not really in a fixed position for much so you can kinda accept more differences i think.

you will probably want your bars a bit higher like they said for CX but you will find out as soon as you start riding around what will be good for you.

If it works, go for it. Personally, I have 3 different bikes that I currently use for different things and none of them fit the same.

It's also worth mentioning that the link to Stybar's bike is 4 years old, and the geo of the Crux has more than likely changed in that time. If he still raced cross, he might not be on that same bike size. Or he could be...

I find a smaller frame to be more nimble/flick-able, but you have to do what works for you.

I generally just go for the appropriate cockpit length on either bike - Top tube + stem. I tend to like a little shorter for racing as the bike is a bit more agile and has a shorter wheelbase. Helps prevent me from locking my elbows too. But its all personal - ride what feels comfortable.

For long distance gravel racing - I like larger, for shorter CX races I like smaller.