I think that you do not want his slaps. of course, all things your Dom puts to you, may not be what you 'want'. What I worry over is that you are not comforted out of the feeling... does he properly look after you after it occurs? Have you talked of how you felt?
I am not experienced, but with face-slapping I could be just being silly (not in Play), if so it is very light, a gentle swat on the cheek. If it is to admonish my pet, to convey the severity and need for punishment, it would be firm - never hard. We have communicated that it sends a very strong message to him emotionally and psychologically, and is not an action I now take lightly.