How much land do you need to feed yourself for a year?
Well if you are good at farming and depending whose numbers you believe somewhere between 0.1 and 1 acre per person (my guess is it's nearer the 1 acre mark if you want a diverse diet and some resilience if some crops fail)
How much is an acre?
approximately the amount of land tillable by one man, behind one ox, in one day!
208.71 feet × 208.71 feet
66 feet × 660 feet (43,560 square feet)
10 square chains (1 chain = 66 feet = 22 yards = 4 rods = 100 links)
4,840 square yards
43,560 square feet
160 perches. A perch is equal to a square rod (1 square rod is 0.00625 acre)
A chain by a furlong (chain 22 yards, furlong 220 yards)
1⁄640 (0.0015625) square mile (1 square mile is equal to 640 acres)
4,046 square metres
0.4046 hectare (A square with 100 m sides has an area of 1 hectare.)
John Jeavons' reckons you can feed yourself with Bio Intensive Method on 4000 sq ft (less then 0.1 of an acre!)  (though I suspect you may need an 8 month growing season to get close to this)
To farm or not to farm...
So as an individual do you make sure you have enough land to feed yourself if there is a massive collapse or do you buy a small flat in an efficiently run city surrounded by agricultural land run by farmers who know how to feed thousands of people rather than a family of 4?