I know that it is a mere miracle of history that Prussia grew to the size it was at all.
However, I ask you, how much larger can it get? That is your challenge. And so no one quips '1871, done' Prussia cannot be merged into any German nationstate.
For the sake of sheer rationality, it -is- allowed to absorb the states that became the North German Confederation(plus Saxony), beyond that, nothing west of the of the Oder barring colonies and any territorial gains in Scandinavia.
PoD no later than the Conference of Vienna. As another bit of challenge, the language of the government must remain German. Other languages can be deemed official on a local level(where in it will be written beside the Gernan).
This also does not mean the German language, in Prussia, cannot be changed as the state ages. As in, slowly picking up foriegn words or phrases, spelling conventions modified. What languages it is mixed with is entirely up to you.
Aaaand go!