I'm asking since I was just trying to tag my own question and couldn't figure out which one I should be using. Is one supposed to be the CSS attribute and the other a more general tag? It's unclear to me. font-size has 209 questions, and fontsize has 301.
The tags should be merged as they are really referring to the same thing. I prefer font-size, but I don't find any difference between using one or the other.
Nowadays, it would not be possible to create font-size, when there is already fontsize. For example, if you try to create an-droid, you get the following dialog when you click on the "Save Edits" button.