How could Poland (and not an intermarium confederation) be a 20th century Great Power following its independence after WW1?
I think Poland at the least needs to be larger. Accepting the offer of Minsk, gaining all of Upper Silesia, getting Masuria, and getting Danzig would make the country bigger. Lithuania too I suppose, though that'd make the country Poland-Lithuania and not Poland.
Lithuania would make Poland larger as well, but I think that if Lithuania were annexed Poland would end up as Poland-Lithuania.
Avoiding WW2 as we know it would need to be a must as well, but if WW2 is avoided then you're avoiding the nerfing if Germany that's necessary (I think) for a Polish power.
Without WW2 and holding Danzig, Upper Silesia, Minsk, and Lithuania Poland could have a population of around ~90 million today I think.