OSM using in Qgis

Hey everyone,
I want to export a map in QGis using Openstreet maps. But everytime I export it labels and streetnames get an unreadable size. It doesn’t matter which extent or zoom I used.
Can anyone help me how I change the size of this text variables?
Thanks, Manuela

Maybe you can find hints about your problem in QGIS at https://gis.stackexchange.com/ ?