For more, see Buffers
Located: File menu / Settings
Set the line thickness of buffers, a value of 1 being the thinnest and 10 being the thickest. The default value is 4.
To use:
Open Settings.
With the General (the default) tab active, scroll down and locate Buffer Size.
Click on the far right side of the field to the right of Buffer Size.
A combo box appears. Select a value from the combo box.