If I could fit a small in any of the other brands which is 55-56cm could I fit an AFX FX90? even though the sizing for that brand is a bit weird since the small is 56-57cm. or will I have to go a size lower which will be an extra small.

 

There is no guarantee it will fit right. All helmets fit different even within same sizes. Some are fitted for round heads others for oblong. Then there may be differences between the size per claim, one small is not the same as another, sometimes even within manufacturers.

That is the reason I do not buy without having actually tried on a helmet for at least 10 to 15 minutes and have the ability to return at no cost and preferably to do so at a dealership where I can get the right size or different helmet either immediately or within a day or two without RO numbers and all.