You aren't the first to complain, and you won't be the last. As far as Auntie can google, iCal simply isn't set up to handle font adjustments. Although Auntie looked extensively under the hood of the application, it seems that iCal hardwires which fonts it uses.
Auntie did find a promising mention of setUsesFontPanel: in the iCal strings, but nothing much came of that -- even when she hardwired an actual font panel into the application. (Hat tip for modifying the compiled main menu nib goes to @viticci over at MacStories.)
As far as actual, you know, advice goes, Auntie suggests you investigate any of the following options:
Reduce your display resolution. Even though the font won't actually increase, it will seem physically larger.
Use your Mac's built-in accessibility features to zoom the part of the screen that iCal is displayed on. Awkward, but definitely a visible improvement.
Export your iCal data to a format that can be read by a more practical calendar software solution and then lose the iCal.
Auntie leans towards the last of these. There are a lot of excellent options out there, like BusyCal and Google Calendar. Got a favorite? Let us know in the comments.
Auntie The T.