It's too long (coming from someone who loves
Andromache,
Cornelia, and
Valentina, to name a few) but I actually like it much better than
Sebastian, though that isn't hard, as I hate
Sebastian. I'd say "meh" or "looks like they're trying too hard" if I saw it, rather than "eew, I hate it." Not really nice, but there you have it. Sebastiane and
Sebastienne are really nms. The problem with all of them is that they've got the slick of
Sebastian and then the frills of -ana or -enne, and that just doesn't fit, the frills hang off the name.
And now, after maybe confusing you with that description, I'd say I'd pair really short names with
Sebastiana, no more than one syllable-
Sebastiana Ruth,
Sebastiana Rae,
Sebastiana Rose (though this sounds... odd. I don't think they fit.) They all start with R and that's a total coincidence. Hm.
Noa