I don't mean something that magnifies in the way normally understood (that it zooms in) but an app that allows you to select a part of the screen and that will then expand that part of the screen all the way to cover the full screen.

  So if you have monitor 1366X768 and you select a part of the screen that is 500X300 it will then enlarge that part to cover the full available screen real estate.

Would also be a bonus if it could scale it so that 500X300 would keep its aspect ratio as expanded, and then just put everything with a black border.

mouse clicks and other events do not have to go from the expanded screen to the actual source window.

Is there a way to zoom in/expand/enlarge part of my Mac's screen to fill the entire screen?

Basically, I'm looking for an app that allows you to select a part of the screen and that will then expand that part of the screen all the way to cover the full screen. So if I have a monitor that measures 1366x768 and I select a part of the screen that is 500X300 it will then enlarge that small part to cover the full available screen real estate.