What's more important is the wall/ceiling/floor surfaces proximity to the screen.
i.e., it would be better to mount the screen on the 17' wall than the 12' wall, since the side walls are further away from the screen.
Also best to not mount the screen too close to the ceiling or floor.
And if you're picky, the back wall (opposite of your screen) would be the wall you could treat with black velvet or black curtains, unless your side/ceiling/floor walls are very close to your screen.
Even if you cover every inch of your room in black velvet, it will still reflect light of course, if it didn't, you wouldn't see the room at all with the PJ on
At 100", a gray might be 'ok' in a small white room when the lamp is new (smaller than your room), but after 200-500hrs or so it might be too dim for the grey screen.