Largest Plausible post-Medieval Holy Roman Empire

[Ciclavex]

As I'm sure most of you know, Carolingian Francia was a massive empire, spanning from the Eber to the Oder, at its largest extent including parts of modern Spain, Slovakia, Bosnia, Italy and everything in between.

My question is not "How to preserve Francia", necessarily, but what do people think is the largest plausible extent of the Imperial framework that developed in OTL Germany (or, at least, something similar), with the whole of it being considered part of a single political Empire with a single Emperor, regardless of the actual power of said Emperor? Could such a framework possibly have included a large part of modern France, as well, under the (theoretically) same sovereign?

 

[eliphas8]

Hungary, Croatia, Flanders, Wallonie, and Champaigne region could be added if you wanted to keep the wank kind of plausible.

 

[Ciclavex]

I've always wondered about this topic; I'm not actually going to write about it (at least, not in the near future; I don't know nearly enough about the period, as my question shows). My question is just about how big people think it could have stayed without the formation of independent sovereigns (like France IRL).