Fret files usually/often come is 3 sizes. Small, medium and large.
The small size is appropriate for standard ukulele frets although they aren't as easy to find for sale as the medium and the large.
I own a Hosco TL-FF1 fret file which is the correct size.
I did buy it off eBay but it may not always be for sale there. Try some Luther supply companies such as Stewart MacDonald or others.