I think Q is closer to the size of the market rather than the size of the "economy." "Economy" implies both tangible goods and buyers and sellers for those good

I think Q is closer to the size of the market rather than the size of the "economy." "Economy" implies both tangible goods and buyers and sellers for those goods. The problem with Q in the Bitcoin sense is that it can move around a lot, since it is an obvious target for speculators with no interest in tangible goods.