Please open Mr. Guard and visit Storage page. Storage page tells how your OneDrive storage is utilized.

The figure in grey indicates the size of the space occupied by other documents – the files uploaded by you.
The figure below is the preferred size that you allow Mr. Guard to use. The usage from Mr. Guard won’t go beyond this setting.

Type in a new value to reduce or increase the allowance for Mr. Guard. Alternatively, you can drag to reposition the slide to complete the setting.

Please note, when the specified allowance is smaller than the current size being used by Mr. Guard, a purge operation will be kicked off. Snapshots for emails came in earliest will be deleted in order to reduce the usage.