I rarely do natural convection simulations, so I reviewed the analysis guidelines in the help section. For the bucket configuration there are two different suggestions for the size of the external volume. The image, below, was copied from the help file.
The sketch suggests a 5D height with a 7D width and depth, where D is the average of the device dimensions.
The text recommends a height that is 10x the device height with width and depth that are 5x the device width and depth.
The recommendations in the sketch would result in an external volume that is shorter and fatter. The recommendations in the text would result in an external volume that is taller and skinnier.
My thinking is that the recommendations in the sketch are more appropriate for a bucket analysis. But I'd like to get confirmation, and also point out the discrepancy.