I agree the size depends on what you are using it for. But for me, the 8'6" is great. I don't like having it in the way and I used to have it on my 310 & I could get by thru the transom door. I couldn't imagine messing with 12' dinghy. Now, in the current davits, it hangs perfectly and is the right size for a family of four.
I agree weaver davits are a great way to go.