They each increase a little bit in length. The first one is about as long as Metal, second is close to Water/Wind (can't remember the second, haha), and the last one is the longest and most confusing (and longer than the Fire dungeon).
Without spoiling how to get into the extra dungeons, I'll say that you should talk to everyone in the final town to get some hints.

Also, if you're up for it, Hobb grinding for the best shield in the game (which will cost you a killer 9999g) isn't a terrible idea. It has 22 def and has a small chance to negate all damage. Not necessary, especially because Hobb farming is annoying as shit, but it IS the best shield in the game. Not to mention, you'll want some levels before going into the extra dungeons as the monsters in there hit harder than almost everything in the game.