The first photo I did in this thread with the plywood facing the other direction it was 4' wide and then a 1' wide piece. But by turning the plywood the other way I had to change that so it would line up to meet on a joist - therefore, it's giving me those odd widths. I'm trying to play around now in SketchUp to figure out maybe spacing the joists closer or doing something so I can have a 4' piece and a 2' piece that will meet on a joist.
You could just put the joists on 24" centers. That's what I did with my coop. I used 2X8s for the joists, and went over that with 3/4" plywood. It's a chicken coop, not the TajMahal. That's plenty for a coop floor.