Look on the cable you have installed in the boat from the factory. I should have stamped or printed on it what type and gauge number, like 6-AWG or 8-AWG.
Now, give you a little insight and then you can figure what you want to do. Lets look at a large TM that can pull 40amps on max. We are going to start off with a good, like new battery that has a 12.6vdc charge on it. Also understanding that a 12V battery is considered discharged when it drops to approx. 11.6vdc.
Also understand when you figure the length of battery cable for the TM it's twice the length you are going to be running it because there are two cables, the positive from the battery to the TM and the Negative from the battery to the TM. The current has to pass through both cable to get from the negative to the positive post on the battery. So, for calculation purposes we are going to say it take 25 feet from the batteries to the TM so that would be 50 feet of cable.
Now, if you have a TM that pulling 40 amps on max, and it's 25 feet from the batter, here's what you get with voltage drop the resistance of the cable causes.
With 8-AWG you will see 11.3 VDC or 23.5VDC at the TM,
With 6-AWG you will see 11.8 VDC or 25VDCat the TM
With 4-AWG you will see 12.1 VDC 25,6VDC at the TM
That's why in another post, I mentioned I run 2-AWG with my 24 volt 80 pound trust TM.
Small cable is like connected a dead battery to it before you even start using it.
Also understand. these figures are when the motor is pulling a full 40 amps. If you are just running on partial power and say only pulling 15 amps through that 8-AWG, there is only a 0.2VDC drop so the voltage the TM will see changes from 11.3VDC to 12.4VDC.
Now, that should give you enough info to decide what you need to fit the way you fish.
I do a lot of chasing schooling fish, stripers, bass, etc so you are having to move in a hurry when that are breaking the water. I want all the ump I can get on max.
If you seldom run of max, it may not be worth the cost of the more expensive larger cables.
I don't have time right now, but I see I need to do some editing and fixing some numbers and get the right figures for a 24 VDC system in here. I was just typing and figuring off my head right quick for this post. Just to give a quick idea.