some go so far as to say 0.88 mm,, about the size of a grain of sand) and expanded to its current size over the past 13.799 plus or minus 0.21 billion years. Volume? Distance photons have travelled since that cosmic explosion.
He thought there was only one Big Bang. Hawking is a believer of Richard Feynman's (left) M-theory, that there are multi-universes, or parallel universes, almost jokingly first described by Erwin Schrodinger in 1952.