kind of surprised that no one else tackled this yet...
the easiest way to figure out what size wire you need is to look at the fuse size of the device.
the dc-dc usb has a 10A fuse on it which means that is the maximum that it will ever draw.
so according to my handy-dandy power chart:
you require a 10gauge wire from the battery for only the dc-dc usb for a approximate 5 meter wire length.
for the amp, i recommend looking at the fuse that it has installed in it, and use that for power wire sizes, as the wattage ratings of different amplifiers can vary widely.
where it might get more complex is when you want to run multiple devices off the same wire. for this, simply add the fuse ratings together, and use that to determine what size wire you need.
of note-- always fuse the wire for whatever the max is-- so if you were to only run the dc-dc usb off a power wire, you would install a 10A fuse by the battery as well.