there is not one specific place that the width is set, it is being effected by a number of different styles, in a few different style sheets and by a number of different selectors that are used in the results.
I'm afraid theres is not a simple fix for this due to the way your page is laid out.
using Fire Bug, you can select each of the elements in the results section to see the CSS that is effecting that element. There are a number of elements in the results that have the width set to 1000px. you will need to edit the width of all of those selectors to change the size of the results.