Try settings > enviroment > view > Message log's font size
And I little extend your question. Is there any possiblity to change global font settings for C::B? I'm not wx developer, but is there possibility to modify some source codes to set custom font (especially size) for whole program? I know that it uses global system settings for GTK, but changing them is not solution for me, becase fonts in other GTK apps are nice. Btw. I am KDE4 user (using QtCurve for GTK2 apps) and problems are with "Project Manager" and "Symbol Browser" font size (or mayby too wide font style).