I've read that broccoli can grow to 2 1/2 feet. Cabbage wouldn't be that tall.

I would consider 2 feet my absolute minimum, and probably go to the 2 1/2 foot height. That would enable you to build the cage out of 6 foot wide chicken wire (2 1/2 feet tall on the sides, and 1 foot wide.) You could make it as long as necessary, and just fold one side over to cover the ends. Then attach your row cover over that to keep the insects away. It will be sturdy enough to hold itself up, although you might want to attach it somehow so that it doesn't blow away.

The only issue I see for you is that row covers are usually only 5 feet wide. That means you would have to use a double wide piece. Another option I've read about (but never used) is bridal toole. I don't know what dimensions it comes in. Mosquito netting is another option, that comes in large sizes.

Kim, to add to what EG said about the Tulle. It comes in 54 inch and 72 inch at the fabric stores. It's easily cut to what you need. It's also WAY cheaper than row cover. Saying that though, if you already have the row cover, I cut mine for early spring coverage lengthwise to use over a 2 ft wide bed. But for bugs, I find I like the tulle much better. Cheaper, light weight, let's the sun and water in better than row cover, and certainly keeps the bugs out. Besides, a side benefit....it comes in pretty colors....You Too Can Have a Prom Garden! LOL

Thank you, I will probably make a couple of row covers and then use BT on some of the other broccoli and see which one turns out best this year. This is the year of experimentation for me. The Prom Garden sounds awesome, I have tulle at home for making my daughter tutu's, maybe I will use tulle on part and I already have row cover from previous years so I will use row cover on another.