The reason the text is disappearing is that you are most likely setting size in pixels, and it is actually using an html specification (just like the output box). So if you do font size=30, you are making a huge font that is most likely exceeding your maptext_width/height. If you want to use pixel sizes, then you can use css. The best way is to set the style-sheet for your map control but you can always do it on the fly with span tags, eg:

Alternatively you can look up the html sizing parameters (they go by relative sizes, so you would do font size=+1 or -1 to increase/decrease from the default).