Wagons can hold up to 7 dirt crates, 9 huge crates and 15 medium crates.     Idea shamelessly stolen from Wess, these things are amazing, please add!   The 500/250 sizes are nice as they are add up easier to 1,000, Making market transactions a little cleaner when making bulk sales rather than trading in partial crates. 150/300 I never did understand how those numbers came about.   The dirt crate is very nice, when you are working on massive terraforming project