English girths come in different lengths in 2" increments.
Virtually all of them have elastic on the buckles on one side and most of them have elastic on both ends.
Between buying a girth in the correct length and having elastic on both buckle ends, adjustability is a non-issue.
The girth I bought must be "suitable for the trails" cause my saddle stays on the horse no matter what I do on the trail, even riding out violent sideways spooks, jumping over creeks, and bushwhacking through deer tracks.
As far as the fleece, it picks up a bit of debris here and there, but it is all on the outside edges so it doesn't bother the horse.
When it gets too grungy, I brush off the horse hair and any bits of leaves and what not and throw it in the clothes washer along with the fleece saddle pad.
They come out sparkling clean with bright white fleece.