No, you'll have to increase the width of each and every dialog separately. Besides, don't forget to adjust the dimensions of the banners, but this can be done in one place:
<WixVariable Id="WixUIBannerBmp" Value="$(var.ImgPath)\NEW-BANNER.bmp"/>
<WixVariable Id="WixUIDialogBmp" Value="$(var.ImgPath)\NEW-BACKGROUND.bmp"/>