If we're going to dismiss personal unions, then "Great Britain" was really only an English empire
Its one thing to critically examine the amount of actual control Budapest exercised over some of the more far-flung portions of the empire under either Louis or Matthias, but to say that because the territorial limits of Hungary itself didn't expand that 'those don't count' is just being pretty asinine.
To the OP, a Hungary that holds Bohemia and Poland at the same time is probably your best bet. Really though what you're looking at here is for the Hungarians to take the place of the Hapsburg Austrians ITTL. You might get a Hungary that rules Cisleithania, Bohemia, Poland, Serbia, Romania, and all of the Dalmatian coast, but in typical feudal form its unlikely to be a centrally controlled empire in the same sense as say, oh, Song-era China, and would be more like the HRE in governance. That isn't to say that you
can't pull an imperial, centrally-controlled Hungary out of this, but its going to take time and plenty of butterflies. Thankfully with a POD so far back you'll have the opportunity for both. If you're just going for sheer size in terms of territory controlled, in some form or another, by Budapest though, then give Matthias a surviving (legitimate) heir. As much as this board loves Louis, I just don't think his situation is as advantageous as that of Matthias', so I'd say the best POD would be for Catherine of Poděbrady and her child to live.