Blue Sea Systems circuit wizard.
http://circuitwizard.bluesea.com/#
Very helpful.
Need to have amp load of the circuit and the length of the conductor in both runs (out and back).
A 150 amp circuit running 40' (20 feet out and back) would require a 4/0 wire.
That seems excessive to me considering the starter is a very intermittent draw.
Might be better to find the amperage of the alternator and upsize from there.
I think I used 2GA on my Sceptre with a 225 2012 ETEC when I moved the batteries to midships from the stern. Only about a 10' run though (each way) and my alternator is rated for 55 amps.
Circuit wizard says I need a 4GA for a 55 amp alternator. So upping it to 2GA gives me some wiggle room. (wizard recommends 0GA for 150 amps)
Starts fast and has had no issues for many seasons. (I think the ETEC came with 10' of 6GA for the factory battery cables.)
Just my .02 cents USD.