Gellar field size

Anyone have any info or lore around the size of a projected gellar field?

I've read a fair new novels and they always seem tkr ever to it as a bubble if real space around the ship. So that implies it's not on the surface of the craft but the barrier is projected out some distance. There's also examples of craft being close enough that their fields merge.

But then the hulls are always sealed for warp travel. I remember they red corsairs in the night lords trilogy being caught on the hull as it entered warp space and being basically evaporated by the currents of the warp. They were at the time walking on the outside of the hull.

So if it projects a bubble when is it not safe to be outside of the hull? What protection does the hull really provide against currents that can melt the very essence of anyone standing on the outside of the hull?