Sorry, no. The CGI.pm method scrolling_list implements a <SELECT> tag, which (according to my IE/NS4 reference) has three attributes: name, size, and multiple. size only controls the height, so it would seem that width is browser-determined.

I would strongly recommend against it, but maybe you could wedge the box in a frame (blech!) if it was absolutely critical to have a constant size. Preferably, however, just modify your dataset to make sure it's a decent size, either by padding an entry or truncating long ones.