Silly question I know, but I just want to know how to measure it.
I believe the measurement is taken diagonally? i.e. from the bottom left to the top right corner of the monitor?
Diagonal is correct.
It depends on whether you are referring to CRT or LCD.
Back in the dark days of yore where we are all just mindless cretins, monitor manufacturers upped the number games by marketing their monitors as 15", 17", etc... and the measurement (diagonally) includes the monitor bezel.
With the rise of the Internet (and hence, the proletariats), monitor manufacturers when they state the dimensions, are now officially measuring just the visible diagonal width of only the screen itself, and does not include the bezel anymore. This is to ensure the senior management does not get lynched by mobs.
the size of the estate also depends on the screen format (e.g. 16x9 or 16x10)