Answer: If you want to grow the maximum in that space, you might look into the approach called square foot gardening, in which spacing is very tight and there is a minimum devoted to pathways. It uses raised beds and includes space saver techniques like using trellis for vining plants such as cucumbers and small melons, and succession planting where early, mid and late season crops can be grown either together or one after the other in the same space. For instance, you could plant early lettuce beneath a summer bearing tomato plant, or grow an early crop of snow peas followed by a summer crop of say Swiss chard. You could also grow vining squash beneath pole beans. For detailed information, you might look at the book written by Mel Bartholomew, it has been re-issued recently. He also has a web site where he describes some of the techniques.
Otherwise, the normal spacing would be listed on the seed packet. Generally you would space plants about as far apart (measuring from the center of each plant) as it should grow wide at maturity. If you have two different sized plants next to each other, you still measure the mature widths, add them together and divide by two. If you plant in matted rows (wider rows about three feet across which is about how far a person can comfortably reach) you will waste less space for walkways.
Finally, try to plant tall things to the north end of the plot so they do not shade their shorter neigbors. With a little planning ahead of time you should be able to grow a wide variety of plants in that space with the exception of corn. Have fun with your garden!
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