If everyone has a copy of the block chain, how much space does it take? Since every wallet program has the entire block chain info. How much space does the block chain take? And what prevents it from growing too big where we would run out of space?
I run a full node, and as of today, the blockchain is 10.1 GB.  Since the bitcoin network is active, the answer to your question about how big the blockchain is something that is quite time dependent.  If you were to come back in a month and ask the question, the answer would likely be 11 GB.

I was wondering a while back about the size of the blockchain and whether or not it would eventually consume my entire hard drive and I crunched some numbers and calculated that if I expect my computer to last another three years (it's two years old now), the blockchain would then be 60 GB.  I have a 250 GB hard drive and with the amount of free space on it (141 GB), even when it gets that big, I should still have plenty of free space.

Nothing really prevents the blockchains from growing so big that we would run out of space, but given that Moore's Law applies to hard drives as well as processors, I'm not all that concerned about this happening.  By the time the blockchain will be in danger of consuming my whole hard drive, my computer will be old enough that I'll likely need a new one anyway.

However, if this is something that you're concerned about, you always have the option of using a lite weight client like Electrum that does not require you storing the blockchain on your hard drive.  Electrum is available at http://electrum.org/

In any case, most people these days have enough storage space to store the blockchain.  The bigger bottleneck is that bitcoin-qt is a bandwidth hog and bandwidth tends to be a lot more precious than hard drive space so if you are using a relatively low bandwidth internet connection, this would be another reason to use a bitcoin client that does not require that you run as a full node and relay transactions such as Electrum.