I would recommend a measuring tape.
Would recommend placing basic components in place. A flexible measuring tape or measured piece of string or rope can give a good idea on the length.
Couple of considerations with wire. Be careful about great prices. Copper coated aluminum, CCA should be avoided. As mentioned above wire should be rated for higher levels than fuses to protect the wire.
If in doubt while more expensive a heavier gauge wire provides a buffer albeit at a higher price. I went over specs on my wire, perhaps OCD but my system runs cool.
If there is any chance of expanding the system buying wire now might provide a buffer against inflation.
Finally regardless of wire sizes make sure that your lugs are properly sized for both the wire and the terminals they will be attached to. This was an expensive lesson for me but was steered in the right direction by this forum before the system went live.
