Select a chart and open the Chart view size tab in the command bar. Tick the "Fixed size" option. This enables you to edit the "Width" and "Height" fields by typing in your custom chart dimensions.
After ticking "Fixed size" in View size it is possible to change Unit from dropdown menu. Options available are "Pixel", "Millimeter" and "Inch".
Open the Chart view size tab in command bar, there tick the "Fixed size" option. This will allow you to adjust Unit, "Width" and "Height" of your chart. Next to this is option to add your custom dimensions to the "Favorite sizes" drop down list by clicking "Add".
This will open additional window which allows to save your custom dimensions as one of "Favorite sizes" and make it possible to use it in the future.