The World Wide Web Consortium maintains a
statistics page about screen sizes.
The main reason they do that is because screen sizes have historically been a big factor in how Web pages were built. It's best if the page content fits horizontally on almost everyone's computer screen, without the need to scroll sideways. There are exceptions to that, like art gallery types of presentations where you scroll sideways through images. But no horizontal scrolling is generally a principal most developers try to stick to for most types of content.
This page has a 1000px wide layout. That provides good contrast with some of the older page designs linked to below that are less wide.
Original Math Dept Page (1996) made by a Lake Forest CSCI student.
This was the era where Web pages were first capable of displaying graphics, as opposed to text only. No one was really thinking much about centering page content and screen widths. The state of the art interactive technique at the time was called
rollover graphics. When you mouse over a rollover graphic, JavaScript was used to substitute in a second graphic. The mouse out event would then substitute the original graphic back in. This era predated the use of CSS.
Next Math Dept Page (2000) made by someone in LIT.
This page featured 600px wide centered content. By this time, the college wanted each departmental page to at least have a common header at the top with links to other college pages like the student directory, etc. The Math/CSCI department still maintained the content of the page. Notice how horrible the cheesy animated GIF graphics that look like leaves are. It was not long after this that the the college hired a professional design firm to redesign the entire LFC site.
Site for My First Book (2000) made by me.
This page featured 600px wide content, but not centered. It looks strange now not being centered, but at the time most people's computer screens were at the most 800px wide so it still filled up most of the screen. About 15% of computer screens were still only 600px wide, so pages were still being designed to fit horizontally on those screens. I made all the graphics in the page myself, including the rollover graphics that make the top menu and the small arrows in the left column menu.
Site for My Second Book (2003) made by me.
This page featured 700px wide content. By this point, so few computer screens in use were only 600px wide that developers were making pages in the 700px-800px wide range. All the interactive mouseover/mouseout effects are done using JavaScript to change CSS background colors. New CSS capabilities were replacing the use of rollover graphics. But the CSS
hover state was not yet usable on all HTML elements, like spans and divs, so the reliance on JavaScript was still heavy. Nonetheless, this was at least starting to look more like a "modern" page you might see now aside from it not being very wide.