The nice thing about being a CD is you can just wear with whatever makes you comfy, which is an option women don’t always have. If you are more interested with passing, you should look at the whole picture when you stand in front of the mirror. If you have a small frame, chances are good you will look better with smaller crossdresser breast forms. This is by no means “the law.” Wear what you like. If passing for real is most significant to you, then you’ll likely opt for breast forms that help your score. If your goal is simply flouting some superbly large breasts, then dimension won’t matter so much to you, apparently.
Another come near to finding the right size of breast forms is to go right down the middle of the road. If you are leaning one way or the other, pick out something “averages,” and see how it works. If you wear your breast forms for a while and then tire of them or just prefer other ones, you can always give them to someone else who can get use out of them.
It’s not critical, and some stores don’t specified in anything but the biggest option or lower pricing, but it might be helpful to find a store where they know what they’re talking about when it comes to bodyshaping, etc. Look around, ask around, and then remember that variety is the spice of life!