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But we try in almost all cases to make sure it looks good in anything from 350 up with no horizontal scroll bar, the only thing that really restricts us being the width of images and a reasonable wrap around of text around it.
The hardest thing for us is making sure that the lines of text do not get too long when people have whopper screen widths. Does not really help readability to have long lines, especially on the web.
We assume that a significant minority do not view the web full screen, but windowed, and that this trend will continue.
Our main 'ideal' is that this is the web, not a constrained newspaper or book width, so pages for us, should really be designed to stretch and compress gracefully rather than be constrained to a certain width no matter the capabilities (or lack of) a persons window to our site.
We do, as a design principle however, favour white space, so we tend to use percentages for the page as a whole (say 95%) and at least one column, and a fixed width for maybe one column, which needs to be narrow to look good, for one of the two to three columns.
But I also take one more step. I keep all the individual columns of text to under 600px so that a 640x480 visitor can access the content by side scrolling just once. Then the entire column is brought into view with one simple action and the user doesn't need to keep scrolling back and forth.
Fluid design is definitely a nice way to go - and is easily achieved by specifying widths in percentages or just allowing divs and other containing blocks to be as wide as they can be.
If your clients don't understand the need for a fluid design, as tedster suggested, then at least make sure your content appears in the center of larger windows, rather than stuck in the corner like the classroom dunce. :)
Using values such as those, no side-scrolling will appear for those using 800x600 resolution. I also prefer that because I run 1280x960 with my browser window covering the left 2/3 of my screen, so thats about 800 as well.
People using 640x480 are becoming fewer and fewer, and they are *used* to side-scrolling because a majority of websites accomodate a minimum of 800x600.