(2nd battery going to first battery--first battery positive goes to starter and the negative is grounded to frame). The batteries are located by the rear axle on the frame so it's about 7 feet of cable. I am wondering if I have big enough batteries and should I go to series 65 truck batteries. Also, I am wondering if my cable needs to be increased in size.
You can check your cabling to see if it's up to task by doing what's called a voltage drop test:
Grab your multimeter, set it to DC Volts at the lowest scale that can handle 14V or so (for most, this is 20V). Hook the thing in parallel to the cable / circuit you want to test.
For example, if testing the negative side of the equation, hook the negative lead of the meter
DIRECTLY to the negative post of the battery, and then the positive lead of the meter
DIRECTLY to the body of the starter or the engine block. Checking the positive side is similar, positive meter to positive post of battery, negative meter to the big stud on the starter where the positive battery cable attaches. If you hook the meter backwards, no problem on most digitals (you'll get a negative reading, though..) but could cook your analog meter types.. And it is CRITICAL for this testing that connections be directly to the battery posts / endpoints or else you're not really testing things.. Here's where those 20' meter leads come in handy.
Once the meter is hooked, disconnect the fuel solenoid so the engine will not start, then crank the thing while observing the meter reading. You should NOT see anything over .6 volts. (Yes, .6, as in just over half a volt...) And that's not the greatest, really. You want it to be less. If it's over that, you've either got bad connections or insufficient cable sizing. You can work your way back to battery or to block with one of the test leads to try to find the bad connection or cable.
When running battery cables, if at all possible, run a cable from the negative of the battery(s) DIRECTLY to the block. (And cables from battery negative to frame and body, or from block to frame and body...) Every connection in the path from battery(s) to your load (starter, in this case) is a two-fold problem: One, there will be loss, even if it's not that great. Two, it's a potential failure point, whether from vibration / fatigue or corrosion.
If you do decide to run negative through the frame to the engine instead, make sure that you use VERY beefy cable from battery(s) to frame and from frame to engine.
And.. Anywhere you connect a cable, be it frame or block, needs to be absolutely clean and free of paint / rust / other impediments. Once you have the connections on, tight, and tested you can paint over them to protect 'em.
I'm going to go out on a limb and say it's likely on the negative side of the circuit that you'll find problems: If you hook up jumpers the way I do, namely direct to battery on the vehicle / battery doing the jumping, positive clamp to positive battery post and negative clamp to big hunk of engine block on vehicle being jumped, then what you're doing is bypassing / augmenting the negative side of the circuit as well as bringing another battery to the party..