Steve McConnell has a chapter in Rapid Development about methods of estimating how big a project will be and how long it will take. He seems to be a fan of function point analysis. I haven't tried that, but it would probably be a good starting place if you want to learn more about how people try to estimate how much work various projects will take.

On a related note, your estimates are likely to be substantially off because of inevitable feature creep. Steve also recommends watching the movie Bugsy. As he says, ...it's hard not to feel a sense of justice when Bugsy is finally gunned down for changing his mind too often and spending too much of the mob's money. :-)