You should reset the browser's zoom to default.
You are addressing an issue that is not directly related to RadRibbonbar, but to browser's mechanism of zooming of specific HTML elements and this is why we cannot help here. If anyone of your users report such a problem, you can suggest him/her to reset the browser zoom to its default value of 100 percents by hitting CTRL+SHIFT+0 (Ctrl+0 in Firefox).
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