Highlight the text you want to make bigger/smaller, click the box that says "text size", and select a size from the drop down list.
If you're using the grey forum theme, you'll need to hover your cursor over the drop down list to see the sizes, since it's currently white text on a white background.
There's gotta be something you're doing wrong, but I can't figure out what.
Until then, I guess you can do it manually. Just type out the size tags, which would look like this:
Code:
[size=medium]TEXT[/size]
Of course, the size doesn't have to be "medium". "xx-small", "x-small", "small", "large", "x-large", and "xx-large" work, too. So do numbers, like "7" and "34".