Test fails on 32bit architectures

[gibmat]

When tests are run on a 32bit architecture, they fail because sizeSuffixMultiplier() in size.go is defined to return/use an int and a 32bit int cannot hold the really huge suffix multipliers. A 64bit int is large enough; updating sizeSuffixMultiplier() to use int64 allows tests to pass on both 32bit and 64bit architectures.

Also, since this code was copied from juju/utils (ugh!), please be sure to fix this bug in that repository as well.