Hi,
I also saw the the painfully tiny font in the Documentation Center search bar after upgrading to 12.1. Windows 10.
Two weeks ago I contacted Wolfram Support about the issue and they responded by acknowledging that it was a problem.
I have Mathematica on two PC's (with different screen resolutions). On one of them the problem was gone within two days of reporting. Font size back to normal. On the other PC it was fixed today. I assume the fixes was done through background updating.
