I don't think it makes that much of a difference, big or small. I'm currently playing around with speaker angle trying to find that sweet spot.
Do you already have front ATMOS-enabled speakers installed? I sit quite far away from my fronts (13 feet), and so the regular 70 degree angle that Dolby dictated wasn't really providing much "height" affect. I purchased some isolation foam kits to modify the angle of the speakers. So far it's improved the effect of the speakers, and I'm getting a bit better "height" effect, it doesn't sound like sound is coming from a front-height speaker.
The more you prop up the speaker the more disperse the sound is on the ceiling though, so I've got a bit of testing to do.