Given the difficulties of styling form inputs of the file type (uploads), you CAN set a width by hacking the cforms.php file... so, my suggestion is to put a field into the section for file upload to add a numerical value (eg. 40 or 56, etc) which would be used in building the html of the input as in size="x"
I've simply added that to the elements used when building an input for upload, but any updates to the plugin will overwrite that...
so, suggesting you add this variable into the upload settings for each form and use it to add the size attribute directly to the tag.