I'm pretty sure they’re the same size as humans, there is more evidence that leads to that side of the argument unless Silksong does something to change it. The Knight should be about 3 feet tall. This is the Knight just before he is about to fight Hollow Knight and the Radiance so he should still be capable of dodging the Radiance’s light beams. I don't really know how we should treat dreams in Hollow Knight, the Knight or anyone with a Dream Nail or Dreamers can physically interact with the Dream Realm, and dreams are able to interact in the waking world. Sounds similar to the Magicant from Earthbound.