It's not too prosaic, it's just too imprecise. There is no standard size, and some regulations even allow a range of sizes.

https://en.wikipedia.org/wiki/Football_pitch

I don't think that's relevant for Register units. Clearly there's a standard brontosaurus somewhere, for instance, otherwise how could we have the conversion table? However, sorry, I was wrong about the football pitch, at least as a unit of area, equal to 194.89nWa per the original article: https://www.theregister.com/offbeat/2007/08/24/so-whats-the-...