Do keep in mind that the banner has to be very visible to the website user so the user doesn't miss it.
That being said, it is technically possible to change the width of the banner by overwriting the 'width' on the "CybotCookiebotDialog" element by using the CSS !important rule.
Please note: This is not something we have tested and it could possibly cause problems with the height of the banner.
Alternatively, if you have some coding skills, you can make use of the ‘custom’ template for the banner/cookie notice (available with a premium subscription) and build your own cookie notice/banner. There is a guide for this here: /hc/en-us/articles/360003921454