You can always choke up on the hammer. Easier than making a whole new handle if you decide you want it longer later
I'm no expert in this realm, but have some relevant experience on other projects.
90% of my files come from garage sales...less than a buck apiece for the small ones.
For this, i would reach for a small double cut, for a smoother finish.
Not too small though, not like needle files...a little wider is easier to keep flat, and not slip of such a small work area.
And get yourself a wire brush, to keep the file clean. They gunk up pretty quickly when working on soft metals