Yeah, I don't typically go for the boring pieces, except by request.
I have had two pieces in my 150. One is 3ft long by 1 ft tall and about 2in thick, very aged chunk off of a tree that had to have been 24 inches around or larger.
The other was just under 4 ft with 9 different spines coming off it.
I am currently gathering small stumps and rootballs. "Small" being anywhere from 12 in base and 14in tall to 22 in base and 24 in tall.
I just sold a piece to a LFS, It was a 8 inch trunk that had been struck by lightening and split about 14 inches down, it was hollow all the way through, about a 4 inch hole, I mounted it at an angle, it was about 32 inches long x 20 high.
All of the stuff I get will float, so needs to either be strapped down, mounted or wedged in.
Usually the slate is covered by the substrate. if not, you can spread aquarium silicone over it then sprinkle your substrate on it and let dry.