Definitely 13 inches. Mine are a little bigger and I like it that way.

This post gave me a new perspective on SFG. The grids on several of my beds are made of lathe and are one solid grid tacked to the top of the frame for easy removal if needed.

The lathe is 1.25 inches wide. So I guess that I am not getting a true 12 inch square since the grid is taking up a few inches of space across the bed.

If you want more squares for different types of veggies stay with the 10 inches. If you are satisfied with fewer plant species go with the larger of the squares.

I say 13 instead of 10. When you figure your planting grids I would still use a 12 inch guide. A little extra room is better than not enough.

I think it really depends on what you plan on growing. If you are planning on planting a lot of different types of small plants, or are you growing larger plants.

For example, if you want to grow carrots, beets, lettuce, etc., you would be better off with the smaller squares (obviously with fewer plants than the normal recommendation) to allow easier succession planting. However, if you are planting larger plants, broccoli, cabbage, tomatoes, etc. They would prefer the larger space.

Personally, I would probably use a little of both.

I would split it mainly into square feet (9) and then use the extra 4 inches to grow small crops that you would normally put 9 or more to a square, just plant in a single row around the two edges.

Good gardening, Mary