Making a full harness from scratch is quite a big job. It's easier if you have an existing one to copy, but even then it's probably cheaper to buy a ready-made replacement. However, if you want to add extra fuses or relays etc then you will need to DIY.
I've never seen a wiring diagram that gives cable sizes, but it's not difficult to figure them out, or you could copy what the original harness used, as steve said.
The rule of thumb for cable sizing is to calculate the maximum current possible when all the connected items in the circuit are switched on, add about 50% safety margin and pick a cable rated for that.
If you're making a harness from scratch, it's a good idea to add extra fuses (the original design is really stingy with fuses), also to add relays in the headlight circuits.