I think that you have to go to the menu at the top and choose Presentaion|Theme Editor|Stylesheet (which is shown on right hand side of screen).
You can change the font percentage in there - e.g font: 62.5%.
This may be incorrect but it's the only way I can see.
That will do the trick - but it's easier to change the defaults in the stylesheet - a quick coding change to the theme - then you won't have to mess about with each post
That will do the trick - but it's easier to change the defaults in the stylesheet - a quick coding change to the theme - then you won't have to mess about with each post