Currently, I run two HDS 12's, and I'm thinking about buying a third. I use separate batteries for running my electronics and starting my engine.

 

I don't know if it's possible, but I'd like to be able to run my electronics for 12 hours/day. Ideally, the size of the battery would be 11" x 6 7/8". Does anyone have any suggestions?

I am by no means an electronics expert but I believe that size of battery will put you in about a group 24.  The HDS 12s draw 2 amps max  3 units @ 2 amps @ 12 hours = 72 amp hours.  Most of the batteries i see in group 24 size are rated for 80 amp hours so I think this would work.

 

I could however be completely wrong

I run 2 HDS 9s, an HDS 12, power poles and a Point 1 antenna off my starting battery.  I've been doing it for 2 years with no issues.  I use Optima Blue top batteries.