I'm working with 10.7.5 and the Cmd+ combination usually enlarges the font size in any app that responds to that command ( such as Firefox). Conversely, the Cmd- combo steps the font size down.
On my system, I find that one application (SpellCatcher) which is designed to interface with all apps displaying type, seizes the exclusive use of Cmd+ for another purpose. So, I must deactivate Spell Catcher from the top menu bar and then Cmd+ will toggle the font sizes in iCal.
That said, it will NOT do anything for the days of the week or the separator lines between days in the month view.
Font-toggle addresses this one part of they graphic dysfunction of iCal's visual presentation, faint, hard to read pale grey lines and small pale day names.
My solution is to use BusyCal which has all the iCal features plus a much more robust adjustable visual interface.
Sometimes graphic design make things look elegant but oh-so-hard to read. What's the point of a really svelt-looking text if you can't read it?