Designing to fit a particular window size is not generally a good idea. When I'm creating a new design, I constantly change the size of the window to check that it looks right at any size. Because some people will have different size screens, running at different resolutions. Some people maximise their window, and others don't. Some people have sidebars running, some browsers take up more space, some people have a larger or smaller default font size.
Using ems and percentages is generally a good way to go about creating a site that is flexible and will fit in whatever window it is given.
Percentages are usually relative to the browser window or to the parent element. So if you have something inside
<body> that you give a width of 50% to, that's 50% of the full width. If you have something inside that that you give a width of 50% to, that's 25% of the full width. This will ensure that your design stays in proportion as the window stretches and squashes.
Sometimes you might want a bit more control than that - eg, you might want to limit the maximum size that the whole page or a particular element can grow to, or to limit how narrow they can go - so use
min-width for those. If you give a size in
ems, it will scale with text size, so you can set it roughly to give a maximum or minimum line length (ie number of characters/words per line). It's all very well having a site that scales perfectly, but if you've got a 3-column layout and someone squashes the window down to 200px wide, that only leaves a tiny, tiny space for each column, which is going to make it near enough unreadable - that's when it's useful to set a min-width override.