I found this which tells actual sizes your TV screens would be.
Just put anything in for the other TV (needs two for comparsion) to get it to figure the results and then you can read what the 42 inch one would be for various display conditions.
As for real size of space used you'll want to look for dimensions in the specifications since screen size is always smaller than the whole TV. Side speaker sets add a lot of width.
I'm guessing that this 42 inch screen is probably measured diagonally. Is this true? If so, what would the actual dimension be acroos the screen, from the left side to the right? I'm trying to see if it will fit into a particular spot I have in mind.