A good rule of thumb is for pouches for blades up to five inches, 7-8 ounce. Over five you can go 7-8 but 8-9 holds up better. Large heavy blades get at lease 8-9 but most of mine are 9-10.
Edges, now that is the bane of most sheathmakers, they can make you crazy. Some simply dont want to go smooth no matter how much you sand. Here's my recipe, your results may vary.
**Even up on a sander, either belt or disk, this helps you get the edge evened up and straight. Then sew.
**Take back to the belt on a light grit, the sewing will compress the leather making it a little uneven, I use either 120 or 150 for this.
**Hand sand with varied grades of paper, 300-600, wetting the leather with a sponge. You are polishing while you are sanding. When it feels and looks smooth your good to go.
**Apply gum tragacanth with a rag and rub vigorously, No burnishing required.
Getting the pattern sized correctly for a pouch is difficult. I always over size and then fit the sheath to the knife during construction.