Actually using certain observation techniques scientists have recently been able to come pretty close to calculating to a pretty good degree of accuracy just how big the universe is an how old... I have a couple of really great episode of a series about this question and more from an excellent series that was on the Discovery Channel called The Universe. You can find them on iTunes, it's about $3 per episode but these two are very worth it.

The two episodes in particular im thinking of are "Unexplained Mysteries" (that they actually do go into some logical explination of some of them.. a couple of them being how big and how old the universe is) and "Cosmic Apocalypse" (this also andresses slightly how big the universe is and how it well end.. truly great episodes I have watched each of them a few times). There is also an episode called "The Biggest Things in Space" which I have not watched but I imagine puts things into perspective quite well and may attempt to answer the big question, and is probably just as interesting as the previous two.

I just now remembered the "Dark Matter" episode also adresses this by explaing measurements made in teh actual size and expansion rates by measuring dark matter and the space between astral bodies. Really cool stuff.

I highly reccomend these shows well worth the money if you have a few bucks to spare. They have some quite renowned and respected professors and reseachers adding to the discussions.