Hi J,
I am also a relatively new member. Installed Victron Multi and LiPo batteries in our Airstream - absolutely love the system. Now building a tiny Transcendental Mediration Studio that will only be solar powered. Basically we need to start with what the maximum amperage draw will be on each circuit. The National Electrical Code ( can find on internet) shows wire sizes for various wire types and configurations. Basically #12 for up to 16 amp on a 20 amp circuit, #14 for 12 a on a 15 amp circuit. I'm going with a DC fuse block rated at 30 amps and then use the small plug in DC fuses like are used in vehicles. For a specific load, like a short string of LED lights you could use smaller wire, like 16, but be sure to install the correct fuse for the wire size.
Ive heard the Square D load centers and breakers rated as QO are also rated for DC circuits - Most AC only devices and breakers are NOT safe with DC.