If you wonder how to get wider select-boxes in WATO on the "Extension Packages - Create new package"-site, you have to edit the file
/omd/sites/"your-site"/share/check_mk/web/plugins/wato/mkpmanager.py - find the following section (around line 437):
return Transform( DualListChoice( title = _("Packaged files"), choices = self._files_choices(), # size = 80, size = 135, rows = 10, allow_empty = False, ), forth = transform_forth, back = transform_back, )
and change the size parameter.
Don't forget that this change will be lost if you update your site.