I know this is a 3-year bump, but I'm hopefully adding something useful...
I've been needing this for a while myself. Like the OP, I'd tried a number of things, and none quite worked. This thread was helpful (filled in the gaps), but I had to read it several times over, because it didn't make a lot of sense in chronological order. I write a lot of guides, mostly as notes to myself, and I put one together for this:
How to Change bbcode font sizes in vBulletin
In addition to simply changing or removing sizes, you can also add some styling with the replacement variables, by inserting divs. I like to add a margin of at least 5-10px before the larger fonts, because they tend to be used as subheadings. I mentioned that in the guide, and I wanted to add it here, too.
I also did not find any font size="7" (4,5,6) tags in templates or in my mods/plugins. However keep that last one in mind -- plugins and mods. Because I do know of some that have size references. I don't think "size=number" is valid HTML, which is why it won't be in any themes.
Thanks, Lynne and mme24.