As a linear measure, a perch (also rod, or pole) is 1/4 of a chain, which is 22 yards (cricket pitch length). A perch is therefore 22/4=5.5 yards or 16.5 feet.
As an acre is an area one chain by one furlong, I'd take a guess that as a unit of land, a perch might be 1/4 of an acre i.e. 1 perch by one furlong.
edit: just wikipediaed it, and a perch as a unit of area actually an area one rod by one rod (or indeed, one perch by one perch) so there are 160 perches to the acre. Another useful fact learned. I think we'll use perches for our next wing area non-dimensionalisation.