A round and rather short necklace is perfect with a straight décolleté strappy dress.
Any necklace, no matter its style or its form, short or long, can match a V-neck décolleté, even a long necklace with a pendant.
A black dress, the famous Little Black Dress, is a reference in terms of pure elegance. With a black dress, all you have to do is taking into account the shape of the dress and the occasion you are wearing it for. To go to work, any colour and any piece of jewellery will do, even a black one. Try colour pearls, ceramic beads or feathers, they are very trendy this year.
With a cotton summer dress, a long necklace or several superposed necklaces and multiple bracelets are recommended for a perfect look.
As to earrings, they can be large or very elaborate when you are wearing a dress. But if you match them with a necklace, they should be smaller.
With a dress, prefer rather fine bracelets if you are already wearing earrings and a necklace.