  We define a universe as the contents of a spacetime box with comoving walls,
large enough to contain essentially all phenomena that can be conceivably
measured. The initial time is taken as the epoch when the lowest CMB modes
undergo horizon crossing, and the final time taken when the wavelengths of CMB
photons are comparable with the Hubble scale, i.e. with the nominal size of the
universe. This allows the definition of a local ensemble of similarly
constructed universes, using only modest extrapolations of the observed
behavior of the cosmos. We then assume that further out in spacetime, similar
universes can be constructed but containing different standard model
parameters. Within this multiverse ensemble, it is assumed that the standard
model parameters are strongly correlated with size, i.e. with the value of the
inverse Hubble parameter at the final time, in a manner as previously
suggested. This allows an estimate of the range of sizes which allow life as we
know it, and invites a speculation regarding the most natural distribution of
sizes. If small sizes are favored, this in turn allows some understanding of
the hierarchy problems of particle physics. Subsequent sections of the paper
explore other possible implications. In all cases, the approach is as bottoms
up and as phenomenological as possible, and suggests that theories of the
multiverse so constructed may in fact lay some claim of being scientific.
