Is there an easy way to increase the width of the list of options for a custom field in the pull down?
Well, I ended up needing to figure this out. So in a file mysite.css I put this:

#select2-drop {width: 350px !important;}


Then in Administer => System Settings => Resource URL, I put the path and file name the file in the Custom css URL.  (All the other places to put URLs on the page want just a directory.  Custom css URL also needs the file name.)