The 39-gram weight is just right for starting out, heavy enough that I actually feel my muscles working but not uncomfortable. After about two weeks of daily use I can already hold them in during a walk without them slipping.
Took a couple of days to get used to the 30 mm size, but now the internal movement is exactly the gentle reminder I needed to actually do my pelvic floor exercises. They stay put during walks and I can feel the muscles working after just a week.
I was surprised how much the 39 gram weight actually makes a difference compared to lighter ones I've tried, you really feel the muscles working. The 30 mm spheres sit comfortably and I can wear them while doing dishes or walking the dog.