Module: Crystal Report designer
File: C:\Program Files (x86)\SAP BusinessObjects\Crystal Reports for .NET Framework 4.0\Common\Crystal Reports 2011\crystalreportviewers\prompting\css\promptengine2.css
In the prompt menu, all the List Box for filtering have fix width. It is possible to adjust this file to make it wider.
This is a global setting, so you can't control a specific List Box size in specific report. Moreover, if the width is too large (say 1920px), the right side of the screen will not be visible. There could be a "height: 100px" that can be added in, but I never try
font-family: Tahoma, verdana;