How do I grow a giant cabbage?

I'm not sure about how big your raised bed is, but a giant pumpkin gets giant. Like 3ft x 3ft. As a kid we always raised giants. What you do is plant them and wait for a likely pumpkin to start growing. You cut off all the other blooms and tend to the one pumpkin on each vine like it's your child. They will sometimes put on 2,3,5 lbs in a day! I'm not exactly sure what my Dad did, but there was a lot of tending, rolling pumpkins, fertilizing and watering. We had a rather large garden and would generally end up with 10 pumpkins around or over the 300lb mark. I'm sure there has to be a great guide for this on the web. Then you keep the seeds from the biggest one, and do it all again next year. We'd always dig out the top part of soil in the pumpkin part of the garden and replace it with compost every few years.

I don't know where you're located, but http://www.team-pumpkin.org/seem to take giant pumpkins fairly seriously. They hold seminars (and make available slides from previous seminars), link to scientific papers on pest management and disease control, etc...Additionally, their "links" page references a forum (registration required) called http://www.bigpumpkins.com/, which seems rather comprehensive, and may be exactly what you're looking for pumpkin-wise. I tried looking for bigcabbages.com, but alas, it may be your mission to register that domain and become the internet's one-stop source for info on growing giant cabbages.

Almanzo Wilder also milk feeds his pumpkin in "Farmer Boy" and wins a prize. There's a description in the book similar to the one at the ehow link.

Ask around at farms / nurseries for seeds from big pumpkins. A family friend grows giants, harvests the seeds and gives them to people who want to grow their own. Mendel, genetics, punnet squares and all that.

Not a comprehensive source, but a fun documentary on the subject http://www.billsbigpumpkins.com/.