Is there a way to set the width of the Customizable Fields Window in a plugin?.
Not currently but I added a new "dialogwidth" option for you. This will
be in 20120801 general release.
So for example, the following can be added to a template or a snippet to
resize its input dialog:
A couple of caveats:
The width shouldn't be set unless there's no other way to get the user
experience you're after. This is so that it can calculate width taking
screen resolution, size of translated text, etc., into account.
The user can resize dialogs and that will override any dialogwidth
setting. You can make sure that dialogwidth is working by selecting
"Reset dialog position and size" from bottom-left drop-down menu.