It is kind of sheet size, and kind of scale.
Say you want to zoom in and print something on 11x17 - if your sheet is set for 1/4" scale then Revit will just center what you want and bascially include everything else it can that fits on that sheet.
If you change your sheet scale (or the zoom % in the print setup) then you can enlarge that onto your sheet.
Or if you do not want it scalable - then just click the "Fit to page" in the print and it will just make it as big as it can to fit on whatever page you specify.
B