What does Boxing Day have to do with boxing?
late 14c., "measurement, size," from Latin dimensionem (nominative dimensio) "a measuring," noun of action from past participle stem of dimetri "to measure out," from dis- (see dis-) + metiri "to measure" (see measure). Meaning "any component of a situation" is from 1929. Related: Dimensional; dimensions.
dimension di·men·sion (dĭ-měn'shən, dī-)
A measure of spatial extent, especially width, height, or length.
Scope or magnitude.