I think that you put it wrong. Not the distance is the issue, but the cultural differences. Distance is a secondary issue IF and only if there are cultural differences.
EDIT: A good example is the USA. Huge land, huge country, and since at the beginning it was an amalgamation of cultures, it worked out fine, they don't need to repress minority ethnic groups, forbid rights and send to prison the "bad" people. Other large countries need to do that even now, because of cultural issues.
Culture should be a key here. And if anything, your ability to tolerate and rule multiple cultures should be tied to your level of central control.
I wouldn't wholly discount distance though. In an era when roads were few and often bad and sea and river travel dangerous simply communicating with or administering distant areas was either impossible or required enormous effort. You as a ruler should have to effectively rule far out areas via semi-autonomous governors.
If we take the empire of Charles V as a prime example I'd say it was the limits of cultures and customs as well as sheer distance involved that made it nearly ungovernable.
At any rate, there is not much example in history of the kind of highly centralized state that you're capable of running ca. 1444.