I don’t think a larger prism will help, Alan. As you suspect, it’s probably due to unstable seeing.
As an aside, I reckon I see as much detail using Baader solar film as using a Lunt Herschel wedge. I do find a solar continuum filter helps (in addition to the solar filter of course), tho the Sun turns green! It also helps to dim the image a bit, which also helps with a prism (could use a polarising filter to a similar effect- if your image is too bright it might make it harder to see detail)
