The display size specification is because the mount extends upwards from the cabinet. The arm height is of a length which is best suited to accomodate displays up to 40" with most standard mounting hole locations.
Because the arm itself doesn't typically move up or down, just the brackets on the arm can move, they have decided upon 40" being 'safe' for recommendation.
It has NOTHING to do with the capability to support a larger display. Heck, if you could find a 60" display with mounting holes low enough to use the bracket, and of the proper weight (most are now), it could work on that mount just fine.
But, I've done some research on these types of mounts and the size is because of the height of the arm, not because it won't support the weight.
So, yes, your display WILL have holes the line up most likely (VESA standard 200x200 or so) and you can definitely mount it if the height gives enough room and you are happy with the final look.
If you do NOT have enough height, you can add some wood blocking and bolt it to the mount and add a second lightweight low-profile mount to get it where you need it to go.