How big where the holes in the Titanic's hull made by the iceberg?

It likely was not just the breach in the hull that was the Titanic’s downfall.

The bulkheads were built so that water could flow over the top into adjacent compartments.

Also there has been widespread discussion that the company bought cheap sup- par rivets which may have contributed to the sinking.

http://www.independent.ie/world-news/europe/cheap-rivets-blamed-for-massive-loss-of-life-as-titanic-sank-1349579.html

Overall the design and materials were a major contributing factor to her sinking.

for a highly scientific video check out http://maddox.xmission.com and search for the post with titanic in it. There is a video about this