In general here are the rules to follow:

Small Plants: 

The small plants are your radishes, carrots, spring onions etc. They are planted 16 to a square which works out to exactly what the spacing instructions say on the back of the seed packets (thin to 5cm).

Medium Plants: 

These are things like your spinach, beets, bush beans, large turnips etc and they are planted 9 to a square (10cm spacing between plants).

Large Plants: 

Parsley, basil, marigolds and swiss chard all fall into this category. Because you are using the Square Foot Garden method, you will constantly be snipping and harvesting the outer leaves of your edible greens like large head leaf lettuce so they don’t take up as much space as in a conventional garden. Large plants are planted 4 to a square.

Extra Large Plants: 

The extra large plants are those that take up 1 entire square by themselves – plants like cabbages, shiso, eggplant, peppers, broccoli, cauliflower and your vine (indeterminate) tomato plants.