No, you can't change font sizes and I don't believe this will ever be a feature. There is no real need for it (other than different header sizes, but this can be taken care of with header, b, u, and i) and it is usually abused. Not abused as in people make huge font sizes or super-small, but rather people use it too often and it throws off the clean cut of the forum that I've worked so hard to keep. (i.e. no images in sigs, all avatars same size, etc)
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