Hmmm...not sure about that one! Really, I suspect the best way to deal with daughter-in-law butt size is to pretend you don't even see it...ever!
Mother-in-law butts, however can be estimated (don't never, EVER, measure it!) in terms of fathoms, leagues, chains, rods, or (for REALLY wide ones - the kind where they're showing off if they're sitting on a single bar stool) parsecs. Any units of measure larger than that might leave one open to accusations of exaggeration...
Of course, there is volume to be considered too, with the possibility of adjacent gravitational changes. One shouldn't mention the inexplicable veering of small cars in the direction of it, or the fact that light seems to bend in its vicinity. Commenting on the frequency of families taking up shade beneath said mass for picnics on sunny days is
right out.
_________________
Scratch building, at continental-drift speed, a custom McSoreley-design framed, dual-Weber 45DCOE carburated, Zetec-engined, ridiculously fast money pit.
http://zetec7.webs.com/