The size of the wire is determined by the load it needs to safely carry. This can be obtained from the manual or the supplied fuse size. Once you have found this value, multiply it by 1.25. This means a 10 amp radio will be 12.5 amps. A 30 amp radio becomes 37.5 amps. A 60 amp radio becomes 75 amps - and so on. This is REQUIRED to prevent the wire being used at 100% of its capacity, which would cause it to heat up. Enough heat will cause insulation to burn. Insulation comes into play below. Use the chart linked below - and use copper wire.
In order to use this chart properly, you need to determine the insulation type. Different types affect ampacities of the conductor. Typical "romex" wire (the stuff in your house) is rated for "60 deg C (140 degrees F)". Specialty wires THW, THWN, USE, XHHW, THHN etc. are rated at "75 deg C (167 deg F)" and / or "90 deg C (194 deg F)" and have a higher ampacity for the same size copper wire than "60 deg C (140 deg F)" wire as a result. If you do not know which insulation type you have, assume the lowest "60 deg C (140 degree F)" type. Look up the amps you calculated above for your particular radio in the "60 deg C (140 deg F)" column, otherwise look it up under the column with specialty insulation types instead. If the CALCULATED amp value is NOT listed, pick the next LARGER value. Follow it back to the LEFT most column to learn the SIZE wire needed with the same insulation as listed in AMP column.
If your radio was 48 amps, it calculated to (48A x 1.25) = 60A. If you don't have the wire, you can go to a store and say you need 20 feet of #4 copper wire with "60 deg C (140deg F)" insulation, -OR - #6 copper with THW, THWN, SE, USE, or XHHN insulation - OR - #6 copper THWN2, THHN, XHHN2 or USE2 insulation.
The values above hold true regardless if in a circuit that is 6 volts or 600 volts. You should increase the wire size (lower #) on DC circuits only if the circuit length is 50 - 100 feet or more. AC circuits can be hundreds or more feet long before increasing the wire size is needed.
I hope this was helpful & good luck. I hope you've got a ham radio license before you transmit with that radio in the 10 meter band.