What Gauge Wire?
 

I want to add a 2nd battery to my V-21. There is about a 7 ft. run between the batteries from side to side in the rear.. I will need a positive and negative cable. Anyone have thoughts on what size cable to use? Thanks.

#2 minimum depending on what engine

And if you don't believe spare, look and see what size you have going to the engine now.
If you haven't been having any problems with your current wire size in the past you need to consider that you are now doubling that wires distance.
For this additional wire going up at least one wire size might be prudent, going up two sizes may be necessary.

You made no mention of a battery selector switch.
If you are using one this may change things a little especially depending on the location chosen.

Hey Spare, If I used 2 gauge wire and only used the 2nd battery for emergency starting( 2nd battery as back up) with the 1994 Merc 150 do you think that would work OK?. I would use battery #1 all the time. It would be hooked up through a Perko selector switch.

#4 is what most carb 2 strokes use up to 10 ft from the engine, I prefer #2 all the way, but it is sometimes hard to get thru the engine harness loom. High quality wire, good cable ends and tight connections go a long ways. You system is only as good as the weakest point, whether it be a bad connection, crappy wire, or a cheap battery switch

Bigger wire with fewer connections