Tested alone, <select class=input-xxlarge>
sets the content width of the element to 530px. (The total width of the element is slightly smaller than that of <input class=input-xxlarge>
due to different padding. If this a a problem, set the paddings in your own style sheet as desired.)
So if it does not work, the effect is prevented by some setting in your own style sheet or maybe in the use other settings for the element.