Hi again Daaaaaaaave, here it is the definitive volume versus length answer!
Volume of a board can be increased by adding length or thickness or both! Then the floatation can be increased by the materials the board is made of. Roughly speaking a PU board and normal resin will give less floatational volume than an epoxy board of the same dimensions.
You can subtract or add floatation by thickening or thinning the layers of glass, effecting strength and lightness. Step decks, loaded domes are attempt to give volume without giving fatarse rails, therebye surrendering power and manouverability in turns. But wait there's more, board stiffness or flexibility for you to consider.
All this is what is tweaked to give the responses a surfer desires at their own level of skill.
When all is added and subtracted you have the functional volume
Here is your answer, neither volume or length are the most important!
it is the combination of each that floats your boat.
The board must be easy to paddle to catch waves otherwise you just be a priority buoy. Length well again, easy to paddle easy to catch nose if long, but if you're learning you'll do that on whatever! PADDLE POWER .
Too much volume and floatation, hard to duck dive and drive under white water corky on turns, ( in other words harder to dig a rail to push the turn.)
Some boards to consider and since your don't need coolth,
Try a 7S of both epoxy and normal construction gives volume, the epoxy is floatier and you could probably go shorter on the epoxy.
There are some mid range surftec boards I have a G&S magic fish as my mid lengther, floats like a long board.
Just another thought the smaller you go the less stable you are going to be in your learning phase you will wobble yourself to frustration especially if you only surf occasionally.
Go to your local surf shop or among your friends borrow boards, figure out your answer, it is different for everyone.
Now stop worrying and go surfing as often as possible on as many boards as you can get hold of