I leave my fabrics all in one piece ... and cut them down as I use them.
This way, you keep the piece as big as possible. As you said, you are kind of designing as you go. What if you take the notion to use that fabric as the backing? (for this or another project) .... the larger the piece the better as if you have cut it up, you may end up with the need to seam it in one or more places to get it back to where you want it.