I have a selection of box cutters, exacto knifes and saws. Mostly I use an OLFA brand heavy duty box cutter. The extended blade is at least 3" long and about 3/4" wide.
I also have what we used to call a keyhole saw, no idea what the proper name is. Just a long multi-purpose saw blade on a handle. I have one that allows for interchangeable blades like a reciprocating saw. Handy, but not for smooth cutting. If I want a smooth, square cut I use a table saw.