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.
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.
You can share the defined sizes with your colleagues by saving them in your department account or company account.