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You can change such these styling following This Tutorial. The Travel Extend Version is Responsive and the Width is defined according to the Screen width. So it is in %, not px. But you can use px value there.
Suppose, you can use the following Code in the WP-Admin > Appearance > Travel Options > General Options > Custom Code within Head Area Box
<style> #content { width: 67%; } #right-sidebar { width: 27%; } </style>
