How big is a barn?

List of unusual units of measurement - Wikipedia, the free encyclopedia

"One barn is 10−28 square metres, about the cross-sectional area of a uranium nucleus. The name probably derives from early neutron-deflection experiments, in which the uranium nucleus was described, comparatively, as being 'big as a barn'. Additional units include the microbarn (or ‘outhouse') and the yoctobarn (or ‘shed')."

And more at the link.