Even among "full size" bicycles you get variation in wheelbases, so I can't imagine that every bike would hit the floor, and I've ridden some small-wheeled bikes with wheelbases to rival my full sized touring bike, so it might be worth asking around to see if there's really any reason to worry that your bike might pose a problem.

Of course space considerations are always a concern, and it might also be a good idea to figure out how likely the route is to be at capacity for roll on service. The regional forums might be the best source of that information.

I agree that the best, catch-all, scenario is to show up with all your gear in one carry-on-legal bag and one carry-on-legal folding bike. The problem is getting your gear to fit into one bag. If you can do that, you'll be in great shape. That's a goal of mine, as well, because is really opens up your options for travel. I am right now trying to set up a folding bike that can fit in a suitcase along with a duffle of all my gear along with the towing parts needed to tow the suitcase. Then I can do it all Bike Friday-style and build my bike from suitcase which I then fill with the rest of my gear. We'll see how it goes.

But for you, if getting your gear into one bag is unlikely to work, then you're back to using roll-on service and two bags. In that scenario, apart from the possibility of there being problems with your bike being too small for their set-up, your situation might not differ from a non-folding bike traveler. So if you really want to collect all possible data and feedback, you might ask the question on the Touring section.