To be honest I agree it should be short. Maybe like a dress with a slight tail? Tumblr the way I see it is about pretty pictures and sorta slutty... in a way however many have a lot of meaning especially when they do the typography.

Ballgown but not as poofy and make it just a tad shorter

I would make it a tube, but cut it a tad short

Either ballgown or mermaid!

Neither.
I think something like this: [link] BUT with your flare and style added on.
Tumblr seems to need something a bit more firm and strict, almost uniform like in shape.
But that's just my opinion.

Bell shape imho. It's a care-free kind of childish site.

I hope to see it soon.

Tumblr should have a short dress~!

I have to say, surprise me.

Can't wait for it