Voltage and Battery Size
 

I believe some of the issues I am having with my Disc II are related to the Battery Voltage. Seems fine because the vehicle starts all the time and drives somewhat ok. What size battery should this have in it? The ones thats in it now does not fill the space. Also what should the Battery voltage be with the vehicle turned off vs while running with alt spinning?

The physical size of the battery makes no matter.
A "D" cell battery and a "AA" both have 1.5 volts, car batteries are the same way.
Look on the top of the battery, it will say how many cold cranking amps (CCA) it has, you want a minimum of 700 to start your truck.
You can buy a brand new battery at AutoZone with 1000 amps for under $100.
I have a Duralast Gold, 1000 amps, 900 CCA and I paid $90 2 years ago.

I just bought a Duralast Gold for $90. It has 850 CCA and 1000 amps. It was the largest top electrode battery that fit in the compartment that was in stock at the time. Also pay close attention that the + and - electrodes are on the correct side.

What should the battery voltage be with engine off and what voltage should I see with the engine on and alternator spinning?

roughly 12-13 off the battery alone, and 14.3-14.8 off the alternator, these figures could be different because all batteries and alternators are different, and keep in mind the weather can affect your battery voltage too, colder temps tend to decrease voltage.

another thing to keep in mind is the voltage of a battery is a constant, and what I mean by that is if you meter a dead 12v battery it should still read 12volts (unless it is completely dead, and then it may not) but you can deep cycle a battery (drain the power out and then recharge it) enough times to the point that it will no longer hold the current (the amps) which is what you need to start the truck. Many times corroded battery terminals are a sign that the battery is no longer holding the full current charge from the alternator.