In the book, it recommends harvesting the leaves when they are 6 to 9 inches tall. That must include the stem, which is how they can grow in a small square. I tried them for the first time this fall and I let them get way too big. Perhaps if they were in a corner, the leaves could spill over the sides of the box.

In the photo link below, 2 plants are growing in the corner.

Here is a link that might be useful: What's Growing On?

I followed Mel's plan and planted 4 per square previously. The Fordhook Giants were too large. The plan assumes you will constantly be picking the outer leaves. If so, they will fit. I ended up planting 2 Fordhooks per square, and 4 Ruby Red per square and that was better spacing for me. This year I've planted Ruby Red and Brights Light 4 per square, and Fordhook 2 per square.

Should mention, if squashed in and not picked, then the leaves and stalks tend to have rot. You need air circulation around them. Too crowded, and the water just sits on the stalk base.

Hmmm...I've always planted 4 per square and they have been fine. It's packed but it's ok. I've grown bright lights for pretty much the last 10 years. I do go out there and snip at them all summer long though...

You could always plant at the higher density to get more young tender leaves in the beginning, then thin to a lesser number when they get too big (eat up the thinned plants of course!) You could do this with other plants too, like lettuce. I plant 5 per sq. ft. (in the same pattern as a 5 on a dice cube) harvesting the 4 corners first, and leaving the center one to grow to maturity. Assuming you have enough seed to "oversow" and eat the younger plants, this works great.