I'm going to change my battery wires to my trolling motor. I'm upgrading to the 24 volt 70 lb thrust Motorguide. Motorguide suggests 8 guage.  Is there any disadvantages to installing 6 guage as in over kill?  Too big?  Thanks to all for any advice. 18 foot bass boat with batteries in the back.

 

8 is sufficient, 6 is better due to less voltage drop but it will be larger. Remember that any length you figure if you use wiring tables needs to be the round trip distance not just the battery to TM 

The only disadvantages I see is the wire will cost a little more and will have a slightly larger diameter (only a concern if space is tight).  I'd go with 6ga.  The physical sizes between 8 and 6 are very close, and marine wire is relatively flexible and easy to run.  Not only will it give you less voltage drop, but it will also stay cooler than thinner wire.  You'll also have overhead if you end up with a trolling motor in the future that draws more juice.

Good advice in both replies above 

6 gage tin plated copper jacketed wire, red and black and don't ponder it any further.

Use the original wire to pull in the new wire.

Tom