That's a real similar size to what I have. When I built mine, someone suggested that I put a 5 foot wall along the 8 foot dimension, dividing the 14 foot section into a 5 and a 9 foot section (or 4 and 10) . Put the batteries on one side of the wall, put the electronics on the other. The theory was that any nasty corrosive fumes coming off the batteries would not degrade sensitive, expensive electronics. My batteries are in an insulated box, so it's a non-issue, but I still like the design very much. I decided to leave my shed uninsulated and insulate my battery box. That seems to have worked out OK. There's room in my big section for the batteries, a generator and a set of shelves, that could just as easily be a workbench. I have used a box fan before to keep the generator cool and blow out the exhaust fumes. I have had no problems maintaining L16 size batteries on the floor.
I don't know why you'd want an extra battery just for a light, it just adds complexity. My light is 120V right off the inverter. Lastly, if you want my advice, forget 12V and 24V altogether. Go 48V. Do it right, do it once and don't have to do it again. At least that what I would have done, looking back.