Tableau enforces a 4,000 pixel size limit when you’re authoring. That is, the user interface does not let you set a fixed size dashboard dimension greater than 4,000 pixels. You're attempting to get past this limit. We do this to help our customers with older GPUs from running into problems. These older GPUs aren't always able to handle Tableau’s rendering pipeline. An alternative to hacking the twb file XML is to create and publish an automatically sized dashboard. In this case, we don’t enforce this limit. (Think of the 4,000 pixel limit as a very short fence that can be easily stepped over.) I recommend that you experiment and see if you can publish a large dashboard (>4,000 pixel).
Product Manager, Tableau Dashboards Team