Thickness of a pencil, width of a dime, or as thick as a human hair
As Michael Harvey commented, the thickness of a human hair or of a pencil is a unit of comparison (although I could find no reference in Wikipedia to an 'informal unit of measurement'). In Britain, 'an area the size of Wales' and 'the size of a London bus', among others, are common informal units of measurement.
You will find other such units here, List of unusual units of measurement, and here, List of humorous units of measurement.