If you go back and edit your text, then sometimes text reduces to near invisible. I have noticed this happening if I put in a hard return (Enter) if I have gone back to a line.
If this happens, open the post, by clicking on [modify]. Then highlight the text and change font size. As Ken said, you have 30 mins to do this. After that, just contact one of us mods to change it for you. The best way is to click on the [report to moderator] button, that way we all see it.
If it is a general problem, then you can adjust your browser by holding down the [Cntrl] key while scrolling your mouse wheel. Scroll one way enlarges the text, the other way makes is smaller.
I hope one of these fixes your issue.