-- Last Updated: Dec-06-12 3:43 PM EST --
I just tried to find the article but couldnt in the 5 mins I have. Some researcher looked into this and determined that with a larger blade will allow you to go faster for about an hour. After that you actually go slower because unless you are in Olympic shape, you get tired and cant maintain the higher level of force over an extended period of time. So if you're sprinting, a larger paddle has advantages. If you're doing a marathon or extended touring, it may have drawbacks. As others have said, Whitewater also benefits from more surface area because you need to "sprint" for a few strokes to make a correction then relax and let the water do the work. If you do some searching you might be able to track down the study I referenced to give you the scientific data behind this theory.
Here's my non scientific experience:
I bought 3 wood paddles from a guy on craigslist for my first real paddles. A Kailoa Nani (9.25" wide) a Wenonah Quetico (8" wide) and a Bending Branches BB Special (8" wide but REALLY long). It didnt take me much paddling to realize that the wenonah was my favorite. I prefer the smaller tear drop shaped blade. The nani was ok, just not my style and I didnt like the BB at all.(Im trying to sell my Nani if you want to buy a large bladed paddle)
Well, mid way through the summer I started racing and ordered a ZRE with a 19 x 8.25" blade (their standard width). I used that for about 2 months but found my shoulders would get sore from over exertion. I found a good deal on a used power surge light with a 18 x 8" blade and its my favorite paddle so far. One reason is that it is the lightest paddle I own, but more so it has the smallest surface area of all my paddles. My shoulders would still be sore after a race but it was not as bad. Also, I felt that I could paddle harder for longer.
Some people talk about "slipping" in the water but I never found this to be an issue (this statement doesn't apply to whitewater or sprint racing). I'm 27, male, fairly strong and athletic, so I feel like if you're not an Olympic athlete, slipping shouldn't be an issue for you. In fact, when I order another ZRE paddle I will order their smallest available power surge blade at 7.5".
As other have said, the "correct" way to go faster is to increase your cadence, not your power.
Bottom line: I like a paddle width around 7.75-8" and recommend most people get something around that for general touring.
Also, this page has a lot of good info: