Scene Scale cm or m

What sort of Scene scale are we dealing with here ? If I set to metres a 2 M cube shows that the default explosion is many many miles in size and yet buoyancy is making it rise very fast… Even when we get hit by an Extinction level Event we would not be dealing with scales like this !! As a default cm scale the sim rises up to about 300000 cm or 2 miles which is ok for a nuke
And Am i right in thinking the sim maxes out to 27MV…This is taking 15mins to Sim on the Titan…is that a reasonable time ?