The answer is over at
vbulletin.com. I tested it and it works. Remember to refresh your browser after the template edit. All posts on the forum will show the increased size. Also, do a test by selecting a different size in the dropdown menu of the editor to make sure the bbcode size tags still work. You'll need to edit both postbit and postbit_legacy templates.
Use Search in Templates to find the code. Use
<!-- message --> as the search term.