I was burned once when I bought an 80 mm fan that was supposed to put out 40 cfm at something like 1 watt. Nonsense! I wanted to cut through some of this and what I have done is to graph Watts vs CFM for 80, 92 and 120 mm fans.
I collected this data from various fan manufacturers for their ball bearing fans. What it shows is there is no such thing as a free lunch – the more cfm you want, the more power you need. It also shows that the bigger the fan, the more efficient it is at pumping out the same cfms at a given power compared to smaller fans.
For example, if you want a 1 watt fan, an 80 mm will get you maybe 25 cfm, a 92 mm 30 cfm and a 120 mm 50 cfm. Although I have not shown it, the 120 mm fan will do this at a substantially lower noise level than the 80 mm fan.
My favorite fan size is 120 mm – lots of cfms at considerably lower noise.
When you are setting up your case and deciding on cooling, take a look at these relationships and you will get a lot more efficient fan power using one 120 mm fan than 2 80 mm fans pumping out the same total cfms – and at a lot less noise.