As you can see if you update your profile, you can customize your default font. To prevent unnecessary stretching of the page and make posts remain as easy to read as possible, the maximum font size you can input is 4. Since the forum's default is 2, this is a rather generous allowance.
This is 4. It is large, while still readable. Using any font larger than this will be regarded as spamming, just like large unspoilered images.
This rule only applies to default fonts, of course. Feel free to use larger font sizes for emphasis, or for titles, but try to limit those to a line or two.
We'd also prefer that you do not use black for your font's color, as it does not show well on the dark skin's background.