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You cannot set the width of the spoiler. I mean Erk or Ccoa could, but that would set it that width for all spoilers. And the code is bbcode, although different forum softwares use different bbcodes.
I wonder why you should set the width at all ... you can't make multiple spoilers next to each other, so you won't need to resize them ... also, it'd add horizontal scrollbars to spoilers with images (which they're often used for), which are a pain in the ass on browsers not supporting horizontal scrollbar mouse wheels ^_^
Well I was kinda thinking...
With my Sig and the spoiler it looks wierd when they're different sizes...
But I had a different reason why I wanted to size them, but I can't remember it...
You don't want to make the sig banner huge or wide to match the spoiler, as that could be generally annoying to members with smaller resolutions. However, you might want to center your signature image. That way it won't look as out of place with the spoiler (which is also more or less centered).