I also suggest that a few plants could actually benefit from more room then suggested in SFG. Such as tomatoes and squashes... try to give them a bit more room for air circulation. Everything else they talk about doing is at a good amount of space and is GREAT for people with little room!!!
I planted tomatoes in my SFG this year. They did okay. I would suggest if possible put smaller tomato (growing) plants in. My Husk Cherry Tomato, Roma and Red Pear did wonderful. My early girl and terrific's did okay. I believe they could have benefited from more room. I also made a BIG mistake. I planted 2 or 3 bell peppers per square. I suggest no more than 2 preferably one. I also washed some of my carrot seeds and beet seeds because my bed is uneven. I have stray carrots and beets in my bed. Build it 4 x 4 as suggested. I did two 4 x 8's to save wood and its harder to weed in the center boxes. Proper planning would bring more production. I was greedy this year, I ruined a lot of plants. I would also suggest trellis, you can never have enough trellis or steaks. I used some bamboo trellis and steaks. I had a bell pepper that the peppers were weighting it down, I steaked it up and now it is stronger and growing really good. I will not plant summer squash in there next year. Because of the VB and black fungus it needs more air circulation and proper pest management. Really pay attention to spacing properly, I promise to do better next year. We had some good cucumbers (more than we could eat) a few beans, squash, zucchini, lots of tomatoes (but no enough to can), peppers (sweet and hot), watermelon, cantaloupe, carrots, beets, and lettuce. I have two 4x8, 2 2x4 and one 2x2. Lots of good stuff on the SFG forum.