Wedding outfit that also slims you down

I need ideas please on a formal outfit for myself that has a slimming effect. This will be for a Spring wedding I'm a guest at. It will be a dressy affair, all the women in hats and so on.

I'm really not good at choosing formal clothing. Can you suggest what would be both stylish and understated but not dull? I would like it to be in fresh spring colors, not dark, and don't want to wear trousers. I don't want to look as if my granny could wear the same thing either.

It may be asking too much to want an outfit that would also help me look slimmer, but that would be perfect. Please add an image if you have one of any suggested looks.

Any ideas would be appreciated. Thanks!