I'm looking to get some more filters for my HV-20. Essentially, graduated neutral density and a few types of diffusion. (I'm not really convinced that the diffusions will be valuable, but I'd like play around w/ them and see how they work.)

I see that most of these filters don't come in 43mm. Some come is 49mm. And all of them come in 52mm and 55mm.

I've been told to go w/ 55mm over 52mm b/c it's always better to go larger than smaller. IDK, as I also remember hearing that some lens accessories can obstruct the AF system and degrade its performance.

Any advice would be greatly appreciated. Thanks much.

My advice would be to purchase the largest size filters you would ever anticipate using.
Or cover the widest lens you have.
Most of my screw-on filters are 77mm.