The rules for which provinces you can select in EIANW seem to be as follows: You can select any province or minor that borders on your home nation or conquered minor or ceded province (but not free state). You can select any province or minor in which you have forces (any counter, any garrison, any blockading fleet--fleets in adjacent sea zone may also work, but I'm not sure of that). You can't select the capital province (for Russia, this is held to be Moscow; you can select Karelia if you meet one of the other conditions).
The only thing that seems a bit unpredictable is free states. Sometimes you can select them without meeting any of the conditions, so far as I can see. But then sometimes you can't. For instance, in a recent game I was playing Russia, at war with Turkey, which held Portugal as a free state. It was available to select, though I had nothing anywhere near it. But then it dropped off the list. I *think* the difference was that another power at war with Turkey blockaded Lisbon; maybe that reserves it for them or something.
Yes, it seems fairly random. Can someone who knows address why you can select some sometimes and not others other times?