Well, besides the whole "rule of three" thing.
I received an order of paper and frames last week and decided to put them to good use. Take a look at the Firefly Startrail family representing the range of print sizes I'm planning on using.
From largest to smallest we have 16x24, 12x18, 8x12 and 5x7. I'm debating if the smallest size should be 4x6 because it has identical proportions. That would also make a nicer mathematical sequence. Hmmm….