Have you ever cruised on any other line before? If not, sometimes a short cruise is a good way to gauge if you are going to really like it. On the other hand, if you know that things like seasickness, etc. aren't an issue for you, my personal feeling is nothing shorter than 7 nights. Now, I realize that is a pricey venture, but my reason is this. Anything less than that and it feels like you are getting off the ship before you have had a chance to unpack. Especially if you are flying in from somewhere.
DCL cruises are expensive, and unless you are pairing a short cruise with a trip to WDW, I think it is a lot of additional money to fly in for less than 7 days.
We are blessed to live near the port, and even still, we felt like 7 nights were over in a blink and that is when we decided that for us, we would likely never do less than a 7 night cruise. We take them less often than many DCL cruisers, but that is in part because we prefer the longer (and thus, more expensive) cruises.
As PrincessShmoo mentioned, this is a great site to get lots of information. If you are looking for something specific, utilize the search box, and that will narrow you down to threads that discuss those things. We are all happy to help with anything we are confident about. The one thing I can tell you is that I have not known many people who regretted a
Disney Cruise, especially with kids.