The size of such a thing is very dependant on the personality of people ruling it, the more badass and capable the head honcho the bigger the thing. If I had to make a realistic guess I would say river Prut in the west, Caucasus in the south, river Ural in the east while the border in the north is a very flexible thing, it would probably be a change from the forested steppe into the forested zone but it would depend on the people they could attract under their leadership. More Finno-Ugric or Balto-Slavic tribes they have greater the likelyhood they expand further north.
On average Nomadic Empires lasted roughy a century and a half before the ruling family was replaced by another and a new empire would rise though there wouldn't be much difference between the fallen and the risen Empire apart from the ruling group.
Since this Empire would have a sedentary base to retreat to and than later to expand from they might even survive to this day in a sort of Jo-joing of their borders until the country becomes developed enough to be able to completely stop Nomadic hordes like Russia did OTL.
This Jo-joing we can even see in OTL with core that created Bulgarians. They were first an independent group apsorbed by the Huns and became part of the Hun core, when Huns confederation fell apart they went independent for a while only to be apsorbed by the Avars, when the first Khaganate fell to infighting they were expelled and went to form the core around which "Bulgarians" were formed.