Yes it is possible, Smooth Scroller is originally designed to only allow height specification to make the images look unstretched. The add-on automatically calculates the best width based on the height of the images specified. However, if you want to change width, which is not recommended, you can do so by browsing through the files of Smooth Scroller and modifiy this file
At line 130:
$resizedfilename = $ih->outputThumbnail2($thumbf,500,$imgheight,''); //max 500px width
change the 500 to any value that you want. This is the maximum width the images can stretch into.
Also at line 153:
<img src="<?php $ih->outputThumbnail1($thumbf,500,$imgheight,''); ?>"
Make sure to also change the 500 found on this line same with the value that you specified at line 130.