difference between copper fret wire and silver nickle one?

is there a size and quality difference between copper fret wires and the silver nickel ones?

Both types of fret wire come in different sizes, so that shouldn't be a problem. Because nickel is much harder than brass, it is also more durable. I just had a refret done on a uke that originally had brass and replaced them with nickel. The brass frets were almost completely worn away. However, this was a vintage uke and who know how much it had been played. My guess would be quite a lot.

I've never seen copper fret wire, either on vintage instruments or modern. Is it even available?
Perhaps you mean Brass?
I doubt brass fret wire is available either, anymore.
On steel string instruments, brass would not last as long as nickel alloy, on a nylon string uke, it may show little wear after many many years.

Brass fret wire still shows up on some inexpensive ukes and classical guitars. As long as you aren't using wound strings (the winding is metal and will wear brass frets as owners of some inexpensive classical guitars can attest) then there isn't anything terribly wrong with brass frets. They do have the advantage of being pretty easy to dress yourself but they also discolor and in some cases can even discolor the wood they are set in (usually not noticed because the wood is dark anyway).

Brass fret wires do have one drawback that can be a real pain, though - if the fretboard dries out enough for the fret ends to stick out a little - which isn't uncommon - it's surprising easy to hook the end on a finger and actually bend the fret up (experiencd talking - old Giannini classical guitar).

John

Since this thread is about copper frets you might be referring to "Evo Gold" wire which is a nickel-free copper alloy. It is harder than nickel/silver wire but softer than stainless steel. It's a good choice if you're putting a lot of wear on your uke.

https://www.lmii.com/products/mostly-not-wood/fretting/fretwire