The Ancor calculator Matt mentioned is about right. Things to keep in mind
are that a 25 foot length of wire is only good for 12.5 feet of
distance..........out and back, right?

And the type insulation is worth considering, particularly if it's
going to be near any heat, oil, sunlight, etc, etc.

Remember also that under emergency conditions (no alternator turning)
there isn't going to be that comfortable 14.7 volts to fall back on, but
something like 12.3 or 12.4 from a good, recently charged battery. Lots of
high class SSB radios will start jumping frequency or channels with 11 volts
at the radio, and the tuner will also be trying to retune everytime you
holler "Heeeelp" in the mic.

Welding cable is worth considering. Fine, cable laid wire with good
tough neoprene jacketing. Terminals deserve good swedged tools, and wicking
a little solder in after the swedging is super around salt water and
batteries.

Old Chief Lynn