Generally a small dome can work quite well, particularly for CF/WA as you stated. You do have to stop down to f/8 or higher. While it's never going to have the image quality in the corners of the shots that the large domes have, results can be quite good.
Larger domes are better overall, but work particularly better at more wide open apertures, and excel at over & unders, something that small domes don't do well at all.
There is also medium-sized domes such as the Zen 170mm, which offer a very good compromise, as they are a section of the larger 8.5" curvature of the larger domes. They still work better stopped down, and aren't quite as good as a larger doem for over & unders though. But they offer the ability to do quite good CF/WA, particularly with FE lenses.
Here's a few examples:
9" glass dome (D800 w/Sigma 15FE):
Manta by Optical Ocean, on Flickr
8.5" Acrylic dome (D7000 Tokina 10-17):
DSC_3616.jpg by Optical Ocean, on Flickr
Zen DP170 170mm dome, (D7000 Tokina 10-17):
Soft Coral at Surface.2 by Optical Ocean, on Flickr
Zen DP100 100mm dome, (D7000 Tokina 10-17+Kenko 1.4TC):
Tomato Anemonefish w/a by Optical Ocean, on Flickr
Hope that helps!
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