Largest possible Prussia (not germany) that manages to Germanize all of modern day poland. The POD has to be between 1200s and 1350s
Did you notice, that Silesia, though under german rule for almost 6 centuries, however always kept a strong polish minority ?
Why ? Because the germanization of Silesia was made mainly through colonization and not assimilation-germanization.
First, to do so, Prussia would need ... not to be prussian and not to behave the prussian way. It is a little bit the same idea as have the english settlers americanize the amerindians.
Second, you would not have time for this and I think it would be too late. Prussia took control of its biggest part of Poland in the late 18th century. The ethno-cultural identitues was then very strong. And I especially think that, in this case they were too strong because Prussia and Poland were also separated by different religions.