The highest amperage for fast USB chargers I've seen is 2.0 Amps. That's pretty rare so I'd go with 2.0 Amps for my first fuse.
Only you can guess the fuse physical size so I'll start with the average one at link.
Here's the physical dimension of this common fuse.
See datasheet 2001.
I have a "CBMOV8" car fast charger for my samsung restore phone. I lost the fuse and need to replace it. oes anyone have any ideas what size fuse fits in there? Need amperage...Thanks