If you want some honest feed back:
Your leather work looks well made, looks good and you seem to use high quality leather.
But how you end your stitching is under par IMHO. Melting the ends and pressing it on the leather doesn't fit the quality of the rest.
If you ride a Harley you don't fit...
I like the colour.
I think it won't be unhandy if the hole is to large. I hate it if things get made and my finger won't fit.
Maybe do a S, M & L-size?
Edit: 18mm would fit me comfortably, 20mm loosely
Just look at a lot of knives and make what you like.
Most important: enjoy what you are doing.
Then after a while you'll find what you like, either perfectionate a hand full of designs or make every possible knife tou can think of.
You need either a broach or a handle with a pommel.
If you choose thw latter, you can drill the material between the pommel and blade from both sides.
Step 1: make a drawing
Step 2: collect the materials
Step 3: just go for it and enjoy the process
Aneal the file, harden the tang but don't temper it.
Hit it with a hammer. If it brakes you have a good chance the file will harden, if it bends take an other one.
And do as Stacy says, reas, read read first.