Could you clarify what you mean by "copy"? Did you just mean text?
Also, are you referring to the Button component or another one? Button components only have the default text style and are automatically set to 11px, so there shouldn't be any issues with changing the text size via the toolbar or inspector.
We did a quick test with the Button component (OS X 10.11.2, latest Chrome and Firefox), but we were unable to reproduce the issue. You may want to try clearing your browser's cache to see if that helps. Stale cache can sometimes cause some visual errors. If you use the stylesheet for CSS, you may want to make sure that there are no classes or IDs that are overriding values set in the inspector.
If you still are having issues, please let us know with the information asked above and a screenshot so that we can isolate the issue.