can i cut a large strip board into a smaller one?
Use a ruler or other straight edge to guide a craft knife. Make many passes cutting a little bit at a time. After the cut looks to be reasonably deep you can bend the board and it will break along the cut. You can use sandpaper to clean up the edge.

If you are cutting across traces you might need to make a few knife passes on the back to ensure the traces are all cut before breaking the board.