I had this problem a little while ago, but didn't find a solution. mPDF (which powers dw2pdf) has reduced support for formatting tables.
I'm currently moving to a XSL:FO based PDF generating system for Dokuwiki; it should be pretty straight forward to modify for table widths (although, it doesn't support that plugin at the mo'). It's currently here:
https://github.com/samwilson/dokuwiki-plugin-xslfo
(Still needs lots of work.)