Ann,
If the skin designer can't figure it out, most likely no one can. Setting the width of a content area should be easy for the original designer, and it's easy to simulate a smaller screen resolution even if you have a large screen monitor.
That said, "doesn't display properly" doesn't explain the problem at all. Is it that the content area is wide enough that scrollbars and scrolling are necessary to view the content? Or are the containers/modules not lining up properly? Or are the background graphics "breaking"? Or is it the text size that isn't correct? Lots of things could happen that would fall under "doesn't display properly".
My guess would be to use the Index Wide skin, but that's only if it uses the ".template_styleWide" class for the content area width. Then you should be fine with any size monitor as the content area will fill 90% of the screen width no matter what size monitor or screen resolution. But, you could run into problems if the widths of the content panes are set to fixed pixels widths. Then, the skin's overall width setting won't necessarily work as a percentage since the fixed content pane widths could override that by creating a minimum width equal to the total of the content panes' widths... and again, the skin designer should be able to figure all this out for you quite easily if it is only a matter of width settings.
Of course, you also have to account for the actual content being used. For example, if you set the skin width to 800px, and place a 550px wide photo in the LeftPane and a 450px wide photo in the ContentPane, well, depending on how the skin is set to handle overflow, you're likely to not have it display the way you want or expect. But, again, the skin designer can help with that as well. As long as everything is taken into consideration when developing a solution, and there can be a lot of different variables at play. If there isn't an easy answer to the particular "doesn't display properly" issue you are having, then I would think the designer would offer to do customization of the skin for a nominal charge to tailor it to your particular needs... unless you're not able to narrow down what the issue is, then they can only guess and hope that's what you meant...