What is the end goal?
Are you aiming to use less wood, or get longer burn times? Any particular volume of wood is going to burn at a certain rate and give off X amount of BTU's, dependent on species, size of splits and how seasoned it is. Longer or shorter shouldn't matter except that if your firebox isn't full.
So....
How big is the firebox of the stove?
If your stove will accommodate longer pieces than the 16"-18" you've been cutting, I would cut them 20-24", to utilize the firebox size to near its full length, to get longer burns. After all, more volume of wood=longer burn times.
but...
You will not burn less wood. You will be loading the stove with more pieces less often, or the opposite would be less pieces more often. Based on personal observations, loading with fewer pieces every couple hours uses MORE wood than loading with more wood every 8-10 hours. The fewer pieces is also not realistic unless your home a lot.
To get more out of less, you need efficiency. This can be done by burning better quality wood (BTU's/cord), better seasoned wood, or a more efficient stove. I also want to add, that I've found bigger pieces burn longer than the same piece split into two smaller pieces, It takes longer to catch fire (especially if its an unsplit log), and to season, but in the end you'll use less wood, and a firebox of big splits will have coals longer than the same volume of smaller splits.
Whether its loaded in length ways or sideways, the volume of the firebox is what it is. Personally, Id rather handle less long pieces than more short pieces. Also, more shorter pieces means more cutting, more splitting, more stacking, more handling. I'm all set with that.