Based on what I have read, Poland could easily have gotten all or most of modern Belarus (including Minsk) during negotiations. I have not read anything to indicate Poland would have gotten most of the Ukraine, but it's possible they could have gotten most of the Ukrainian territory they lost with the Second Partition of Poland in 1793 (which excluded Kiev). So I think the Poland of 1793 could have been restored within rough borders.
If so, the idea was that the Grand Duchy of Lithuania would have been restored within three cantons. The first around Vilnius which would have been Lithuanian, but included the Polish majority in Vilnius. The second around Minsk (Belarus-Lithuania for ethnic Belarussians). The third would involve diplomatic negotiations with Lithuania (capital Kaunas) to incorporate it into Poland. Presumably most of the Ukrainian territory would have become a fourth ethnic canton.
With two parts of the old Grand Duchy of Lithuania in their possession, Poland would have turned their attention to Lithuania as it stood in 1920. Negotiations would have been heated. I think there would several possibilities - 1) Lithuania remaining outside of Poland, 2) Lithuania being forcibly incorporated into Poland, 3) Lithuania voluntarily agreeing to be part of Poland with varying scenarios of autonomy and perhaps special status compared to the territories Poland already controlled. I could see any of them happening.
In any case, this new Polish state would be much better economically and defensible than OTL's Poland. Control of Minsk and additional railroad lines would have aided economic development of Poland's eastern territories. Poland would also have some nice north-south flowing rivers to act as a defensive border.
Both Belarusian and Ukrainian nationalism would have been incubated within their cantons. Since the object of the Polish government would not have been to Polonize those regions, it might be possible to accommodate the nationalists of those ethnic groups within citizenship of the federated Polish state.
I also think Poland would have been able to make strong arrangements with the Baltic States (Latvia and Estonia; even Lithuania if it remained outside) on their mutual defense and economic cooperation. Some kind of defensive alliance and customs union might be possible.
Such a large Polish state would be a much harder target for Nazi Germany. Even if the Nazis still thought they would win, it would take longer and without a fast conquest, they would not imagine an easy fait accompli. Even in a potential division of Poland with the Soviets, Moscow would be more reluctant since they could expect more resistance then IOTL. Then again, with a much larger Poland incorporating Lithuania somehow that gave them access to the sea, the Polish government might be willing to compromise with Hitler on the Polish corridor in exchange for continued non-aggression pact. Easy predictions after 1938 are hard to come by.