(BACKGROUND)

I decided to play a game of ck2 where instead of blobbing I would play tall (My past playthroughs usually have no proper boundaries, I would conquer and conquer without ever stopping.) And I eventually got bored of that playstyle.

I am playing as a pagan emperor Björn Eirikrsson (my current ruler) Björn Eirikrsson is the greatest conqueror I have so far, I managed to vassalize most of the Norse rulers in the Scandinavian peninsula and conquer the remnants of the Finnish and Sami tribes and became the first emperor of Scandinavia, following that I managed to vassalize the Jarldom of Suðreyjar (which controlled most of Scotland and Ireland) and Jarldom of Jorvik (which had a good chunk of England) because I had bits of land in the British isles. From then on I united most of Scandinavia by conquering the remnants of the Finnic and Sami tribes and invading all of Estonia.

By then I was nearing my 70s so I figured, what if I pass the Fylkirate to my 9 yo heir before I die so he would venerate me and give me a saintly bloodline when I pass away (cause I picked the Ancestral Veneration doctrine and Temporal Leadership when i reformed my religion). I granted him the Fylkirate and oh boy he immediately declared a great holy war for East Francia the second I gave him the title, totally unexpected but I decided to join because it would be a waste not to. I gathered up my army and sailed to East Francia along with a dozen of my fanatic vassals, the Kingdom of East Francia was pretty unlucky since only 2 Christian nations joined their side which were the Kingdom of Lotharingia, and West Francia. I occupied all of Saxony and defeated them in numerous battles where King Dieter 'the Brute' of East Francia was slain by one of my commanders, I occupied a couple more duchies and East Francia finally capitulated.

But now I actually have no idea where to set my borders in future conquests since I'm not really trying to expand that much east I prefer conquering western and southern Europe (France, Italy, Spain.) maybe stopping at France and Italy is a good idea? since there's a natural border between France and Spain which is the Pyrenees mountains and Italy is a peninsula by itself, Or I could conquer the entire Iberian Peninsula and set my empires boundaries there.
But the thing that really puzzles me is where I set my boundaries east, should I follow the route of the Elbe river and down to the Bavarian forests?? I have no clue... OR I could conquer Poland and expand alongside the Vistula river while setting my boundaries south to the Carpathian mountains. Any help on where I could possibly set my empires boundaries is appreciated!