Help me plant! Spacing concerns.

I am not sure this will work out. Specific concern is tomato placement.

Back (4th)Row: BigBoy Tomatoes, BigBoy Tomatoes, Celebrity Tomoatoes, Celebrity Tomoatoes

3rd Row: Cherry Tomatoes, Red Peppers, Green Peppers, Grapes

2nd Row: Onions, Cucumbers, Cucumbers, Blackberries

Front (1st)Row: Zucchini, Zucchini, Squash, Squash

Not sure what size these plants will all grow to and how much space their roots need. Also I built my planter box with 1x6 Cedar so is 6" deep enough?

I suspect that will be an interesting 4x4.

What should I change to make the space fit better?

Which of these is too big/etc??

I am going to let a more experienced gardener answer that. One question, though: Do you have any open space around your SFG? Is it all open yard, or up against a wall or something?

North side is a wood fence. Open yard elsewhere. I expect I'll need some trellis.

Your squash is going to take up a lot of room. They make a big bush. I made 2x2's just for them.
Grapes and blackberries may be happier separated too. They will get big.
The cucumbers need something to climb. They are vines. Unless you have a bush cucumber.
The 6'' deep is fine.

Grapes and blackberries are more like trees - they should be planted in one place because you can harvest them year after year. They actually may take a few years to give peak production. The zucchini just plain get huge. And their leaves are gigantic. I also use a 2X2 space and I have them surrounded by corn and pole beans so the zucchini can have the lower canopy and the corn and beans can have the vertical canopy.

The tomatoes and peppers should be fine, just keep up with pruning suckers from your tomatoes to keep them trim.

You do want all of your trellised items on the north side and together for convenience sake.

You are, however, making my hungry.

So, maybe like this:
South
Back (4th)Row: BigBoy Tomatoes, BigBoy Tomatoes, Celebrity Tomatoes, Celebrity Tomatoes

3rd Row: Cherry Tomatoes, Red Peppers, empty, empty

2nd Row: Onions, Green Peppers, empty, zucchini

Front (1st)Row: Cucumber, Cucumber , Squash, Squash
North

Your back row is fine as long as it's on a trellis. One zucchini plant will take up a 3x3 square all of it's own unless you try to trellis it also (Mel has instructions on the main SFG website). You could plant it in a corner and let it just have three squares and overflow the box. A vining squash is planted 1 per 2 squares. If your cherry tomato is a determinate (bush) then it will need 4 squares. If it's vining it will need a trellis. Cukes and vining squash need a trellis, so at this point it looks like you will have a trellis on at least three sides, which I can't see working as you wouldn't be able to reach everything, plus the inner veggies will be shaded. If you haven't already built the box, you might consider changing to a 2x8' since you have so many vining plants. Or if you have the lumber to make a 2x4 vining box in addition to the 4x4. That would give you room for cukes and vining squash and smaller plants in front of them.

Oh, and yes, 6" (actually 5-1/2") is deep enough. Just be sure to buy quality compost -- some of us are struggling because our composts didn't have enough nutrients.