I have an 18” toothed bucket and 16” trench bucket. Both work great in my clay. A 12” may have trouble with clay sticking IMO. A 24” toothed bucket would probably be fine for your soil. I think a 30” toothed bucket would be too much.
I'm finally (after over 12 months of waiting) getting my BH92 backhoe in a couple of weeks. I'll be running it on an MX5800. I'm going to get a 12" trenching bucket. I'd like to get a bigger bucket for digging holes, but I'm not sure what size to get. 24"? 30"? I'm mostly curious what others out there have, what type of soil you have, and whether you're happy with that combination. We are mostly clay around here with a lot of large rocks.
A 24" toothed bucket should work for you, 30" is too big.
If you get a 12" bucket for use in clay, you will quickly learn to hate it.
Get the 16" bucket instead of the 12".
The 12" will plug itself up VERY rapidly!