"A good figure" is formal, polite, and just a little bit old-fashioned. You could use it in any context. "In good shape" means fit and healthy. "She has a good shape" could mean the same thing as "she has a good figure," but it is just a little bit informal or disrespectful. If you look at the cover of "Shape" magazine, they are using the word both ways at the same time. "Figure" deemphasizes sexuality, "shape" emphasizes it. It's like the difference between "lady" and "woman." "Shape" is not bad language or taboo, but whether it is OK or not depends on to whom you are speaking and how it is said. It would be safer to use the word "figure."

She is in good shape = She is fit and healthy "She has a good shape" suggests that she is curvy and has an attractive body form. It is an unusual sentence to hear out of context. You would have to be discussing body types for this to really be used and applicable. "She has a good figure" is more usual.