battery and amp in the back now? What size (if any) fuse are on the amp?

Option 1) IF the amp is REAL close to the battery, AND you can access the on-board amp fuses easily, you could consider not using an additional fuse.

Option 2) If you want to be extra safe, you should use the same or higher value than the total fuse rating on the amp. IF the amp has a 20 amp fuse, then you might consider either a 20 or 25 amp fuse in the main fuse holder.