bluesaint
500+ Head-Fier
Honestly, I got lucky. A larger diameter circle will have a greater effect but will over dampen the sound and it might be too bassy and loose. Too small, and you don't get the benefits. I haven't bothered changing the size as it does what I wanted and I don't want to keep unscrewing the grills as the screws are into wood and will loosen each time that's done. Feel free to experiment if you want!
FYI, the template I used was a Hifiman retaining ring using the inside of the plastic ring to make the circle.
The inner circle I'm pointing at here? This is my Hifiman HE6 pad