First of all, I'd get the latest patch. CTDs can still be annoying, so autosave every year is a must, at least for me.
If you want a kingdom thats quite large but still manageable, that's England. Being on an island is pretty handy in order to keep your rebelious kin close to you and delay foreign armies from landing (plus sea trips cost money).
Your treasury should be built up gradually, so no rush to tax those underlings. Later on I tend to keep the scutage at a minimum, thus allowing larger armies to be assembled and keeping loyalty high.
After the first few hours you'll get the hang of it and see that actually it's not a really complex game. Hell, my last campaign ended with the King of England owning 80% of the British Isles provinces, the vast majority of the Middle East up to Baghdad,all of Iberia and the Maghreb, south Italy, a collection of german lands, 90% of the Baltic lands, most provinces on northeast and east Black Sea, the steppe lands north and around Khwarezm, 5-6 scandinavian provinces and Iceland. And all this by 1280 or so...now I wouldn't call this a challenge
Still a very, very enjoyable game
(until the rebellions start to kick in )