Does working my upper body make my bigger or smaller?i want them smaller so any advice would be good?

It will make them bigger because working out makes your muscles bigger. If you work your pecs, the underlying muscle will get bigger. The best way is probably to lose weight, though its a little late for that. At dress strores they often have things that are basically fancy tube tops that can go underneath your dress. You could also just go with a high tube top type dress rather than a low cut one.