OP messaged me to ask about FPS in larger and smaller monitors. I respond publicly so the forum can be of help to others:
The thing that impacts FPS is the resolution you play at, not the size of the screen. You could play at 1040 x 680 on your new monitor too (assuming it can run a lower resolution).
I don't have any knowledge of your computer specs, however FPS should be a secondary concern in most situations. I do think that 18" is abnormally small for gaming -- most players play at at least 720p. For FPS, I would recommend playing at a minimum of 720p with the graphics settings turned down instead of playing on a smaller screen-- so you still get the scope of view that other players get.
I personally play at 1440p and ~80fps, but I spent enough on mine that I should be in the top percentile :-)