Are you looking to leave the city lifestyle, buy your own land, and build your own home? Do you want to have a more self-sufficient lifestyle and grow most of your family's food? Do you want room to have chickens, pigs, goats, something else? If this sounds like your dream, then how much land do you need?
If you're like most folks, you probably want as much as possible. You want room to roam, and you don't want to feel fenced in by your neighbors, right? The only problem is that land isn't all that cheap, especially not in areas that are still relatively close to the city (hey, even if you can make do without Starbucks and Mcdonalds on a regular basis, it's nice to be close to medical facilities and the like) and in an area where it's easy to grow food. Assuming you don't have unlimited money, you may be asking if you can afford enough to do all the things you want to do, raise livestock, grow food, have a workshop, hobbies, etc.
The good news is that you probably need less than you think. There are people who grow 80% of the food their whole family needs on an urban lot (for inspiration, check out the Dervaes family--they live next to the freeway in Pasadena and grow enough food to sell to restaurants on top of feeding the family).
By using space-intensive gardening methods, you can grow a lot of food in a totally organic and healthy way. Check into permaculture, micro eco-farming, biodynamic farming, and square foot gardening for starters.
Ultimately, how much land you need to buy will depend on the size of the family you want to feed, whether you hope to start a business and sell extra produce, whether you want to plant an orchard, how much livestock you want to have, etc. etc. etc. But, you'll probably find that you need less than you think you do. You can do an awful lot with just an acre (and it's easily to maintain than a huge farm!).