Instead of changing the font size, can you set all your calendar colors darker? This would at least provide more contrast between the white lettering, and the colored bars.
If you click on the calendars in the left-hand window, the info box is where you can set the colors. I found the easiest way to assign colors is in the crayon box, since there are way fewer choices than on the color wheels. I'm using Tiger, with iCal 2.05, so the Leopard instructions may be a little different.
Jeff