re: Society breaks down and you have to make it in a subsistence agriculture situation

If you're not doing it now, you wouldn't be able to get started fast enough if it happens right away. How long do you expect to have to survive from the farming? You're asking questions so I'm assuming you're not serious.

You'll need equipment and tools, animals...etc. Do you have a stash of seeds? You'll need that every year. What about salt, sugar?

Check out some farms nearby and go visit. A small plot in town is supplemental, but can't be relied on for enough to feed you unless you have a big hoard of basics.

The larger your farm is, the more you can produce to offset different pest losses and other crap that happens.

In other words, get the largest farm you can afford, with a pond for your fishing.