A warm welcome to Mac-Forums.
The answer depends on which Operating System you are running (perhaps you could tell us please).
The following applies to all windows in El Capitan and macOS Sierra (can't recall if it was so in Yosemite):
At the top left of the Tool Bar on all windows are three coloured dots or circles.
Clicking on the Red one closes the window (and in some cases, quits the app; but not always).
The middle one (amber) minimises the window to the Dock.
The Right one (green) has a dual function:--
1. Clicking on it opens the window in Full Screen. The top Menu Bar disappears, but if you hover over it with the cursor, it reappears. To Exit Full Screen, hit Esc, or move the cursor up to the top Menu Bar and the green dot can be clicked again; or Menu Bar > View > Exit Full Screen.
2. Holding down the Option Key changes the function of the Green circle - a "+" appears and clicking on it opens the window and allows you to drag it to any size you want. Once set, that window should continue to open up to the size you set.
If you have an earlier OS, the red & amber circles fulfil the same functions whereas the Green circle allows you to alter the size to your liking (as in 2 above), but there is no Full Screen option.
Hope that offers some help.
Ian