If it is an image or a rendered view on a sheet then you can click it and sizing handles will appear or you can just change the width and height figures in the properties window.
If it is a perspective view (which I am sure you mean) you click the view on the sheet then click 'activate view' on the Modify Viewports ribbon. You can then use either of the methods described above, to resize it, after which you click outside of the viewports extents, ensure the View ribbon is active and click, viewports>deactivate viewport.
Hope this helps.
Click the viewport on the sheet and hit the size crop button on the ribbon. Make sure the scale button is selected and then enter the size - it will be in inches, unlike Revit's default of feet.
Rick, thanks for pointing that out. I had not noticed this before, but why does it not work for renderings or images, weird!?
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