Attention spans are short these days. You've only got a few seconds to convince someone to stick around and explore your website. If you make that difficult for them, they'll likely go away, and fast.
One of the biggest turn-offs for viewers is having to click alot, and scroll alot, to see what they're interested in. So, how much of your website can viewers see without scrolling side-to-side or up-and-down? That depends, natch, on how big their screen is.
You might be surprised, I was when I first used this tool on my own website.
It's a Firefox add-on called Web Canvas (by Methodologie). When you add it to your Firefox web browser it superimposes a transparent grid over whatever website you're looking at that maps out the browser sizes of different kinds of users. That way you can clearly see, for example, what part of your website 50% of the users will be able to see, what part 92% of the web users will see, etc.
I was surprised when I first tried it to see that only 50% of web users could see the entire top part of my site without scrolling. I changed my website and now 92% of web users can see the entire top part. That's important because if most people are anything like me, they want to click less, scroll less, view more, and know quickly whether or not they want to stick around a website.
Try it, you'll like it!