Label Size in QGIS

Hi,

me again.
I am desperately searching for a method to lock the font size of the features displayed on the Open Street Maps base map in QGIS. Every time I zoom in the font gets so small I nearly can’t read it. And when I print the map on A4 I barely can read anything.
Is there a way to make the label font bigger or better readable?
Thanks guys!
Phil