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.
http://www.amazon.com/RoadPro-2AMP-5PK-FUSES-0/dp/B001JT3HNS/

Here's the physical dimension of this common fuse.
http://www.cooperindustries.com/content/public/en/bussmann/edison/products/ul_fuses_generalpurpose/UL_1_4_x_1_1_4_fuses/small_dimension_electronicfuses0.html
See datasheet 2001.
Bob