1. Measure it diagonally, making sure you only cover the viewable display (the screen itself). Alternatively, plug your serial number (located underneath the keyboard) in Apple's support site. It should tell you precisely what model number it is.
2. Ideally, you will want to get your product number. Some iBook G4 14" models use the 1" 20-pin connector (M9848LL/A iBook G4 14.1" 933MHz), whereas others use the 1.5" 30-pin connector (M9419LL/A iBook G4 14.1" 1.2GHz; M9848LL/A iBook G4 14.1" 1.42GHz)