Use the Clean tool () to remove small holes from a mask.
To clean a mask, select the tool on the tool palette and click anywhere in the mask. All masks of that type will be cleaned, unless you clicked inside a region.
The Clean tool has a single option:
Strength determines the maximum size of the holes that will be filled.
In addition to the Clean tool, you can choose various Clean commands from the Image menu.
Cleaning will only fill unmasked holes, it will not remove small masked areas. Nor will it fill unmasked areas along the perimeter of a mask.