Well, a sunken road needs to be deep and wide enough so the horse can jump down, take a stride and jump out. A coffin needs to be narrow enough so the horse will jump the whole thing, but shallow enough that he wont get stuck in it should he get his legs in it.
I would do it for a normal one stride. If the horse has a shorter or longer stride that's up to the rider to account for. Its a permanent jump, the competitions arent going to change it for each horse, they expect the rider to make their horse fit in the right stride. That's why riders walk courses. If its available to you, go to a farm that has one and measure it to see what they have. My best guess would be a typical one stride fence at 24'.