I have an eight inch stackable dado set, and pretty much any dado or groove I do is about 1/4" deep x 3/4". Once I went to a 1/2" deep, but that was a long time ago. I suppose in this case, size doesn't matter…........ Jerry (in Tucson)
I think most of us have 8" sets because they are the most common. Truth is, most dadoes are relatively shallow cuts and the 6" will probably cut everything any of us need. But you will often see saws with smaller motors suggest a 6" set. That said, I used an 8" set on my Delta contractor saw for quite a few years and never did any damage.
An 8" will have a higher surface speed, but allow for less cutting force and will take longer to get going (and keep going) on a light powered saw. Depending on how much you plan on using it and what quality level you're looking for either could work. For higher quality you will find more options in the 8" size than 6".
Either should work well, depending on the specifics of the dado set in question. I've never had a power issue with an 8"set even with my lowly Delta 36-600…it was a Freud SD208 set. Sometimes 6" sets are cheaper, but not always. It's worth buying a quality dado set IMO. Decent sets for < $70 are rare, whether 6" or 8".
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