How big was a medieval acre?

[Ray T]

Dimensions of an acre are a chain by a furlong (or 22 x 220 yards) accounting for the rather odd 4840 square yards to the acre.

"Furlong", of course, is an abbreviation of "furrow long" so an acre was the approximate size of a mediaeval strip field. You can still see the remains of these in the landscape today and they are typified by their "S" shape. They weren't this shape originally; the shape developed over the years from ploughing as the oxen were turned at the ends of the furrow. 

[brecor]

If the Irish acre is 7,840 square yards, then 620 Irish acres would be 4,860,800 square yards which works out to be 1,004.30 "modern" acres.

I think!

I was assuming that these figures 7840 was 4840 could not be used as they were only defined as such in 1878.

[Koromo]

The Irish acre was traditionally larger than the English acre long before Queen Vic defined a statute acre.  The following is from The Universal Cambist, and Commercial Instructor, Volume 1, page 220, published in 1811:

[Koromo]

By the way, those numbers above held true at least in 1811 (and by tradition probably very close to those numbers for two to three hundred years before that):

30¼ Irish acres  x  7840 = 237,160 square yards

237,160 square yards  ÷  4840 = 49 English acres

[brecor]

Thank you, Koromo. That clears everything up!