Gravel bike sizing

I’m new to the forum so I apologize if this question had been addressed--its about sizing/bike fit.   

I am looking at getting a new gravel bike.  Nothing certain yet as I am still trying to figure out what I'd like. However, its been a long time since I got my first CX/gravel bike--its a 2006 Motobecane Fantom Cross (52 cm).  Its been a great bike and was very affordable for what I was "allowed" to spend back then. I did a lot more gravel riding this past summer/fall and want to upgrade.  I have two road bikes which are also 52's -- a 2005 Salsa Campeon and a 2015 Trek Domane Compact. I was between a 52 and 54 for the Domane. The 52 looks and feels a little small, but by adding a slightly longer stem it has about the same feel as the Salsa as far as the 'aggressiveness' of my position on it. The measuring points for the "52" are very different between the Salsa and Trek being that the frames are different styles, but thinking back, I wish I would have went with the 54.  I've been checking out various companies  'guidelines' for their gravel bikes and they put me on a 50-52. Bikes I'm considering are Trek, Specialized, Framed, or KHS. However, I looked at what Motobecane is offering now and one of their higher end gravel bikes (Mulekick carbon) sizing guidelines puts me on a 46 cm which seems small (aluminum of the same model has me on a 50 cm). I contacted Motobecane and they explained the reasoning behind compact frame sizing of their carbon gravel bikes.  They referenced Giant as the background as to how compact frames came about and why they size their carbon frames the way they do. They assured me that based on my height and type of riding I do, a 46 cm is what they recommend.  I hadn't considered Giant, but then looking at their guidelines I'd fit a 'small' - which is around 45 cm so that's pretty consistent with what the Motobecane guy said.  I'm wondering if anyone out there has experience with these differences and have gone from traditional frames/sizing to a smaller compact frame?  I haven't test ridden any yet newer gravel bikes yet, but am trying to figure out the ballpark range of the size I may need and don't want to have any regrets.  Its hard to wrap my mind around going from a 52 down to a much smaller size, but is it really smaller?  I know I should test out a couple, but again, I want to know a little about what to expect.

What they are telling you makes no sense at all to me.

“compact” geometry has been around for decades (basically any bike that does not have a horizontal top tube like they all did most of the last century).

I look at only one measurement (for starters) when sizing a frame.  That is the length of the cockpit.  Everything else is adjustable (just make sure they are not too far out of whack).

In my case, I need a 64cm cockpit.  A standard bike with an effective top tube of 54cm + 10cm stem gives me a 64cm cockpit.  My last bike takes after mountain bike design as it has a long top tube and short stem.   Oddly, I took a “small” size, but that has a 550 TT + 80mm stem.  It’s a little tighter and lower than my standard, but I’m loving it.

So – if you like the cockpit length of your current bike – if your riding it and the handlebars block your view of your front axle – then measure that and keep that with your new bike.  As long as the handlebars are not too low/high, you should be good to go.

If you are on the cusp between frame sizes, keep in mind that a smaller frame will be a little more nimble, a larger frame will be a little more stable.

Hope that helps.