Hi Jacob,
The Panel or LargeButton of Ribbonbar does not support setting a fixed width. The LargeButton automatically calculates the appropriate width according to the length of text, and the Panel width automatically adapts to the maximum width of the control arrangement in the container, which has reached the optimal space.
Now you can try to adjust the LargeButton placement order or the Text content to see if it has the effect you want.
If you have a new requirement, please open a ticket through our support portal at https://www.appeon.com/standardsupport/newbug.
Best Regards,
Peter