Here's how a 'width coil' works. It's a plastic piece, with a bunch of wire wrapped around it. that's all it is. The wire has a resistance of like 1 ohm or 2 ohms. So, if there's no 'slug' inside the coil (the part you adjust) the horizontal deflection circuit has a huge 1 or 2 ohm resistor in it. The way it's wired into the circuit, if there's no slug at all, the picture is the widest it can possibly be.
The 'slug' is a little piece of ferrite or whatever. It's a certain size and length. If you screw the thing down into the middle of the width coil, it shorts the coil out. So now instead of having a 1ohm resistor, you have a dead short, so it's essentially like you connected the width coil together and got rid of it. This makes the picture as small as it can be. So for instance, if you have a width coil that's screwed up, you can actually solder together the two sides of the width coil with a jumper, that shorts it as 'narrow' as it can go. Keep in mind we're sometimes only talking about like an inch that it can adjust.
So. in your situation, you want the picture to be as wide as possible, because your screen is too narrow. IF your B+ is set correctly (120 or so, have you checked it?) then just turn the monitor off, and using your tool, completely remove the ferrite core from the width coil. That will make the coil as wide as it can possibly go, which undoutedly won't be too far since your pic is so narrow right now.
Check your B+ though, if it's low, the screen will be too small.