Is there a way to use Smart Albums or Automator or something else to find all photos that have a width or height of less than 240 pixels?
You can actually use Spotlight.
Open the Spotlight window. Choose to search in your iPhoto library. Add 3 rules :
For example, if I have "Pixel Height" set to 321, and "Pixel Width" set to 241, Spotlight will turn up all images that are between 0x0 to 320x240 in resolution size.
For your case, both will be 241 then.