of or relating to an empire.
Britain's imperial era
of, relating to, or denoting the system of nonmetric weights and measures (the ounce, pound, stone, inch, foot, yard, mile, acre, pint, gallon, etc.) formerly used for all measures in the UK, and still used for some.
In the imperial system, 36 inches are 1 yard and 1760 yards are 1 mile.
(of a size of paper) measuring roughly 762 × 559 mm (30 × 22 inches).
The paper size is imperial folio, that's almost as big as today's standard A2 page, which is equal to four standard A4 sheets.