Tack vs. fully welding is probably not much of a concern either way. If it was only tacked, it could "leak" a bit, but that amount would be so trivial I doubt it would matter. I would assume fully welded to be better as long as it can be done without warping, which it sounds like yours is fine.
As far as the opening, you want the opening at the end of the rf plate to be at least the same size as your firebox to cook chamber opening. There are some different thoughts on this, and someone with more experience will probably answer, but anywhere from the same size as the firebox/cook chamber opening to a bit larger and you should be ok.
Best way to tell is to smoke something!