I don't know of any way to change the labels that are actually on the task bars, but if you use View -> View Options... to move the information to the left or right of the bar, you can increase the font size.
I'm not our resident OmniPlan expert, though, so it's possible that someone else will hop on later and correct me. If you want a guaranteed response from an expert, send a message to the
support ninjas, and they'll get back to you. :-)