Consider the size of the universe. You might
have read in the popular science literature that the universe has a radius of about 10 (or
20) billion light years. The size of our observable universe is based on Hubble's
Law. Hubble's law represents the expansion velocity of the universe as:

*v=H*_{0}r

Where *v* is the expansion velocity of the universe at a
distance *r, *and *H*_{0}is Hubble's constant.
Now, since nothing can go faster than the speed of light, we can estimate the age
of the universe by looking at our formula and noticing that *v* increases as *r*
increases. Or we can deduce that r cannot be large enough to bring *v* to a
value greater than the speed of light. In other words:

*r=c/H*_{0}

Gives this as the maximum *observable size* of the universe.
This works out to be anywhere from 10 to 20 billion light years in *radius*
(see also "How Old is the Universe")
If we run time backwards with this fixed radius, then, matter would expand
until it filled this volume. A meter, then gets much closer to the size of the
universe... In loose terms, then the size of the universe "ruler" at
the time of the "big-bang" would be the same as a material ruler.
Furthermore, the evidence we've collected over the last 30 years or so indicate that the
the "big-bang" might have been a hot "big-bang". So, our
initial conditions for the big-bang involve a very large, very hot "sphere"
of primordial stuff. Now, if we run time forwards, we can kind of visualize
the beginning of our universe as a very rapid collapse (the inverse of "Inflation"). A similar
kind of event happens when a star (hot ,spherical, massive, dense...) becomes so dense and
massive it collapses in on itself and forms that mysterious black hole. Here's an
interesting question to explore... "Did our universe begin as a star that underwent a
gravitational collapse?" I.e., do we live in a black hole? (See also
"Is
the big bang a black-hole?" for the other side.)