Which piggy bag fuse to use

Hi

I'm thinking of fitting my old TomTom into my Fabia but want to use a piggyback fuse so I can hide the wiring, I hate loose stuff/rubbish or non necessary stuff in my car.

I've noticed there's full sized and mini fuses in the interior fuse box, the question is, which one do I use so it's on a switched power source, amps and size, so I can order the correct piggy bag fuse.

I've never used these before, do I connect the black earth wire wire to a metal fixed point somewhere on the dashboard subframe.

Thanks in advance for any replies.