Kubota L4610 HSTC, International 2400, Hesston 1280,
I think mine are about 9", but we made a 12" plate which bolts on with 4 bolts, and we can pick up dirt. Leave it on 90% of the time, but can take off if wanting to pull roots. We like that we can push, level, dig, when the plate is on.
If you have huge brush maybe 12 inch, if small stuff maybe 6 inch... A lot of small stuff falls through my 12 inch spacing.... Might have to modify it and add a bar in between existing to be really effective.... Yes these are DIY brush forks but same concept, don't think I would go any wider than 6 inches if I had to do it over.... 3 inch would hold even smaller stuff...
I agree that it's really about purpose. I believe at one point EA was making a grapple where you could add bolt in tines to tighten the spacing if needed (Not sure if they still do). It really comes down to the level of detail your trying to clean up.
I bought a WR long and at the time the dealer failed to mention I had options. It comes with 8" spacing. I could move large piles of debris, sod, etc. but rocks fell through that would still be an issue for a tiller and it wasn't that good at separating things. Built a removable unit that drops the spacing to 3" and now it is useful for picking up firewood and sifting reasonably large rock from soil. If you are moving logs and boulders or short on lift capacity maybe go wider, if you want finer screening go shorter. 12" would be pretty useless to me.
Kioti CK3510SE HST, Ford 3400, Gizmow ZTR, Simplicity 7016H
Right now I have 12" spacing and lose a lot of the smaller stuff. Up here where its sandy soil and I'm scraping up a lot of brush, vines and limbs, 3" would be ideal. 6" would probably work down where you're at.