I'm completely obsessed with the Elsa Peretti bottle collection lately. I'm debating between these four. I don't live near a store (otherwise I would go in and compare/contrast) so any guidance on these would be great.
Mini sizes (on 18" inch chains):
Square
Round
Small size (on 24" inch chains):
Square
Round
Jug
Note: I'm not sure if I can rock the 24" (I'm petite in height but yet busty) - I don't usually wear necklaces longer than 18".
18" is a really good length. I would get the round mini - it looks more like a bottle. The smaller size would be better for everyday - the larger is more of a statement piece.
If you are getting a polished silver piece, it can really shine against your skin and you don't need such a big piece for it to show up.
Just to compare, I recently bought this similarly shaped piece on an 18" chain (post #22):
http://forum.purseblog.com/the-jewelry-box/pick-a-pendant-865523-2.html#post27170543
It is only 12mm wide and I believe your round mini bottle is 20mm wide. This little pendant pops! I am not petite but not busty either, so the 20mm on you may be more proportional - I was going for a small everyday necklace that wasn't tiny like a diamond but not too big to be a statement piece. The 18" chain makes it easy for me to wear it with v-neck and scoop neck tops, so it lays against my skin and looks framed by the necklines of the tops. The silver shows up a lot more on skin than it does over clothes.
For my next piece, I was debating between a curved or rectangular chunky metal piece by the same designer with a 1/3 carat aqua stone.
I decided the curved piece would be worn A LOT more, which is a reason I would go for the round bottle if I were you. It goes well with most necklines and feminine body curves, whereas a square-ish shape would be more limiting, I think. But the square bottle has softer edges than the rectangle I was considering, so it's personal preference, I guess.