Do you mean in land area, regardless of how long?

Let's assume Europe stops at the Caucasus, Urals, and Constantinople.

The most obvious candidates which come to mind are the Roman empire, the Hunnic empire, the Carolingian Empire, Charles V's empire, the Napoleonic empire, Nazi Germany and the USSR and it's satellites. I think the Roman, Carolingian and Charles V's empires controlled visibly smaller territories in Europe.

Now what is "control" really? Napoleonic France didn't militarily conquer all of Europe, but was so dominant that it's neighbors were submitted to it, in the form of the Continental system for example, not to mention it defeated most continental forces militarily in subsequent wars, and its army at its height could and did expand from Spain to Moscow. In that sense, it's control was perhaps larger than Nazi Germany's. However let's keep in mind how vast the European parts of Russia are, so the USSR's dominance over the eastern half of the European continent may be bigger than any other empire has ever had in Europe.