Land: How much is optimal for "cost avoidance" income?

Roughly speaking, gardens can be ordered by a hierarchy of size. And, a natural consequence of their size and the law of diminishing returns is what is likely to be grown in those gardens.

The very smallest urban gardens might be just a few pots or buckets on a balcony. The plants are likely to be high-flavor herbs, spices and maybe some enhancements to a daily salad. Chives, rosemary, sage, mint, cherry tomatoes, green onions, garlic, hot peppers are likely to be found in these gardens.

Those needs become saturated for most families somewhere between 32 and 100 square-feet and other foods start being grown. Maybe a zucchini or cucumbers. Perhaps full-sized tomatoes or radishes. Some greens like lettuce or maybe green beans.

The typical "hobby" suburban garden might be about 400 square-feet. Things like pumpkins and winter squash start to sneak in. Maybe sweet corn or potatoes. Perhaps enough tomatoes are planted so the gardener can preserve them for the winter soup-pot.

According to Steve Solomon, author of Gardening when it Counts, the next step is approximately 2000 square-feet. This depends on the climate but this is probably a good number for England, Tasmania, the Hudson Valley, Columbus Ohio and the Pacific Northwest. This size is large enough to mostly decouple from supply-chain shocks. 

The back-yards in this suburban neighborhood in central Michigan are almost exactly 4000 square-feet.

And then in the next breath, Solomon suggests that 4000 square-feet would allow the gardener to "fallow" one half every year while actively working the other. An example of fallow would be to plant red clover or buckwheat or turnips to build organic matter (and feed your rabbits and chickens) in half of the 4000 square-feet while actively growing "crops" in the other half.

Then, if you are the kind of person who plans, you might plant berry bushes beside the paths between the garden and the hen-house and put a few small fruit trees along your poleward property line (north if you are the northern hemisphere and south if you are in the southern hemisphere). Or, if you are in the deep south, maybe you plant them to so that they provide partial shade during the hottest parts of the year.

Scale 

So, while the word "land as an investment" might create images of hundreds of acres in the minds of most rural, North Americans...having authority to be lord-of-the-manor of as little as 4000 square-feet of garden can be pivotal to enabling autonomy for a family of two. Point-being that 4000 square-feet is 1/10th of an acre.

If the 2000 square-feet that is being cultivated was planted entirely to maize, then it could produce enough calories to supply two people with 1200 Calories each per day for a year. Not enough to survive but a huge boost to any diet under austere conditions. If planted entirely to potatoes, 2000 square-feet could produce 2.7 pounds of potatoes for two people each per day for a year.

These calcs assume 150 bu/acre yields for the corn and 40,000 lb potatoes (all grades combined) per acre. For reference, the average corn yield in Indiana was 203 bushels/acre last year and the US average potato yield was about 45,000 lb per acre.

Income

A random village east of Kyiv, Ukraine. Look at the gardens!

One form of income is "cost avoidance". That is money or time or miles on your vehicle that you were able to avoid because you had invested in an alternative that allowed you to NOT spend those dollars, lose time or drive somewhere.

It gets trickier to put a value on it when it is an option or an insurance policy. But it is still a legitimate form of income when circumstances force you to activate those options.

Under conditions like a civil war, it is almost impossible to put a value on the investment because you would bid your last dollar to feed your granddaughter if she was crying from hunger. 

The devil is in the details

Decent soil of sufficient depth. Either reliable rains or infrastructure to irrigate or subsoil moisture within easy reach of the roots.

The plot not shaded by buildings or shade trees for any significant number of hours a day.

No HOA or Zoning busybodies poking their noses into your business. 

A frost-free growing season that is nearly always more than 120 days.  

Excellent weed control (many are edible). 

Varieties that are good fits for the growing conditions. That will likely mean planting some hybrids, at least while getting your feet wet.

Enough gardening savvy to know when to plant and harvest. A willingness to use fertilizers, at least to get started. A willingness to put in the hours every day to make the garden work.