QGIS Helpful Expressions & rules

Rule based labelling

--Apply in the 'Label with' part of the properties

if( $area > 1600,  "field_name_for_the_label" , None)

if( $area > 80000, regexp_replace( "name",'[(](.*)?[)]','' ) , None)
-- which means if the calculated area (project settings) is over then use the "name" column to label if not then no label,
--also strips anything within a bracket from the label

--Apply in the label size field from the expressions button 

CASE WHEN $area > 100000 THEN 15 ELSE 7.5 END

--which means if area is bigger than this give it larger label