If exporting a plot to an image file, additional options appear in the Export dialog.
Check the Keep aspect ratio box to keep the original ratio of width to height as adjustments are made.
Press Reset to return the settings to the original values.
Adjustments to the image can be made based on pixels or print.
For pixel adjustments, use the Resolution pull-down to specify the dpi setting.
For print adjustments, select the units to use from the Units pull-down.
Make the desired changes to the Width and/or Height values.