If you're wanting to change the default width of the nicklist for all channels, you don't need a script for that.
The upper-left context menu of the channel has a menu choice for 'nicklist', but even though it doesn't mention the width of the nicklist nor have an input box to specify the desired pixel width, if you select 'set as default for all channels', not only have you set the left/right/autohide settings default for all channels, but that also sets the default nicklist width too. If you then have a channel that doesn't conform to this new setting, then you'd need to use the same menu choice in that other channel to 'use default'
