Okay as far as i understand it the main reason for this fuse is to protect the wiring not the amp if you have a crash or grounding with your wires underneath or anything else that can damage the wire insulation can cause a short and if that fuse ain't there then at best you can discharge your battery or at worst fire and explosion. As to rating most shops should be able to point you in roughly the right direction it need not be too exact as the fuse on the amp should protect that.

Hope it helps