The one way to change the regular text is buy the Custom Design upgrade. This will allow you to change it once for all.
The other way is to use coding in the Text (=code) editor. But this has to be done on a post by post basis. To change the font size of a complete post you'd add this at the very beginning:
and this at the very end:
100% is default size: change the number to increase it.
Bu this whole idea is a bit questionable, since the perceived font size depends on screen resolution. You obviously have a high resolution screen (as I do). What looks small to you (and me) will look ok to a visitor with a lower resolution screen, and what will look ok to you if you increase the font size may look too large to said visitor...