I'm trying to make a Nexuiz map of a real world building using NetRadiant. How does the scale of the brushes in NetRadiant correlate with real-world measurements?

I'm trying to make the map as close to real scale as I can, but would rather not do it by eye if I can do it with math

its often said that 1 game unit = roughtly 1 in.

but also consider that speeds in nexuiz is a far cry from real world speed. Simply running in a straight line, no bunny hopping or anything makes yah move in roughtly 35 km/h by this scale. (you can easily get to 1000 qu/s, 90 km/h with trick moves)

As long as the relative measurements are equal to the real world building (1qu = 1in, or 1qu = 1cm, or anything else),
you can scale the resulting map with q3map2 (-scale <N.N>) to the right size.
http://en.wikibooks.org/wiki/Q3Map2#General_options

Well, with a scale of 1.0 eight quake units is one foot, or 1qu = 38.1mm exactly. Of course, things tend to appear smaller in-game, especially in first person view.