You need to understand the purpose of the fuse. The fuse is will melt if there is a short that generates more current than the rating of the fuse. That will keep the wire and electrical components behind the fuse from receiving a level of current that would damage them. In the case of the wire, it will prevent the wire from heating up and potentially causing a fire.
You want to have a fuse that is rated slightly above the normal current draw of the electronics behind it. If your vehicle PC draws 12a (as an example), you'll want a 15A fuse. If your vehicle PC draws 18A, you're young to want a 20A fuse.
Chances are that the 20A fuse will be fine, but you really should measure the current draw form the PC to be certain.