Not too many planes make it above FL450 so call it 21 million cubic miles of airspace.
Big Sky works well over Montana but the problem is most of those planes want to go to MIA/south Florida, ATL, the nightmare airspace from BWI to BOS, ORD, DFW, LAX or the SFO/OAK/bay area. Say there are 10,000 airliners/freighters/corp. jets in the US (just guessing.) So you'd think everyone gets 210 cubic miles of air but you're lucky to get 3 miles in trail at a busy airport.