Ok, so FBX provides FilmWidth and FilmHeight. This is lens width/height in inches. When I convert filmWidth from inches to millimeters, rounded, I get 36mm, which matches blenders default camera
Math.round( 25.4 * FilmWidth )
Which will give you 36mm. Same as camera exported from Blender.
I think FBX is providing this data pretty straight forward, Three.js is just not making use of it.
Edit: kept wanting to say FilmHeight. I mean width.