After my first test import from vB3, it seems the SIZE BBcode tags are not rendering properly. For example…
What do I need to do, to get the SIZE tags to perform their function and disappear from the body of the message? Also once I perform whatever change is needed, do I need to Rebuild Thread Information or some other function?
Your issue is that with XF size uses size=1 whereas your import reveals usage as size=-1 (notice the - ) which you will need to replace in all posts so that it is formed properly for the way that particular bbcode works by default within XF.
Take a backup of your db before you start trying to do replacements (you will need some basic regex skills) just to save yourself the time of re-importing if you mess up.
Then run a pattern which is suited to your needs from within the addon's options page in the XF ACP, there may or may not be an example of what you need in the corresponding thread for the addon mentioned above. If there is not an example which fits your needs you will need to ask for help in that thread or hire someone who likes regex (like me) to do all of your post replacements for you.
When editing a single post manually it should parse pretty much always if it is formed correctly and with a screenshot (a small one at that) it is difficult at best to tell you what to do as I don't really know what is there to begin with. However seeing as things look right in the preview I would guess at it being related to caching.
Thanxx for the offer, but for the time being I've got the forum pw-protected by htaccess to keep out all the bots & scrapers until I can get it closer to being semi-ready…plus, I'm not great with htaccess. When I get closer to going live (i.e. after much more is cleaned up), if it hasn't resolved I'd be glad for you to take a look at it.