When we first made the Release planning feature, we discussed this problem and came to the conclusion that Sprint-level selection makes more sense than Story-level selection, in that in applies in most cases. Although I do not doubt that there are exceptions, generally I would tend to say that Stories that are to big für one Sprint are too big in general.

When you select a Sprint for a Release, it basically means that all Stories that were completed in the Sprint are part of the Release. In practice, of course, you could not release that version at all or disable some of the features in it (or they could not manifest themselves, because they are not visible on their own). Still, they are, somehow, part of the release, are they not?