Font size can vary for reading depending on how it was created, what background it's on, and if it's a bold font or a delicate font.
The best way I've found to do it is to generate my journaling, then look at my page at 25%. (this especially if you create in 12x12 and print at 8x8, but the general idea can work for 12x12 to 12x12). If you can read it easily at the 25% size, you're good. If you have trouble adjust up, if it looks too big, adjust down.
Most people find between 12-18pt is good, but there are so many other things that effect the look of how a font reads, it's best to check to make sure.