Hi GeorgiosAthanasiou,
I want to isolate the issue for us to pinpoint the exact concern. Is this issue specific to Excel office app only? Have tried opening other office apps and check if the same problem occurs? Kindly check this article: Customize the ribbon in Office to gather more information about personalizing the ribbon in Office apps. Please take note that it's possible that the settings of your PC's display resolution might have been changed because it’s indicated in the article that it’s not possible to change the size of your ribbon, or the size of the text or the icons on the ribbon. To change the font of the menu bar you can follow the screenshot below for a step by step instructions on how to do the process.
If the issue persist, feel free to update the Office suite and check if the issue will be resolved. If after updating the Office app and it's a no go, you can repair the Office suite through this link.
If you have further questions, simply reply to this thread and I will respond back as soon as I can.
Bob