Since some food products can also be converted into Booze, I often think about food in terms of "Plump Helmet Equivalence". Plump helmets can be grown year round to provide both food and wine; other crops (depending on growth speed and processing yields) can be seen as counting for more or fewer Plump Helmets.
1 Plump Helmet = 1 Food or 5 Booze. Since a Dwarf eats 2 food and drinks 4 booze per season, that's a food cost of 2.8 Plump Helmets per dwarf per season (pretty close to 3), and so about 12 Plump Helmets for a year, and so 1200 plump helmets grown over the course of a year to feed and booze 100 dwarves.
How much land this takes depends heavily on the skill of your growers. Assuming your growers are all rubbish, we can expect stacks of just 1 PH per farm tile. Let us also assume your farmers are slow and busy, so you only get 10 harvests per year. With a little head-math, this leads to the figure of needing 120 tiles of farmland to support 100 dwarves. However, this assumes your farmers totally suck, and so can be taken as a MAXIMUM value.
A good rule of thumb for farming is 1 tile feeds 1 dwarf. However, with good growers, and growing crops like Quarry Bushes (which are processed to give 5x their nominal yield; 5 units of leaves from 1 unit of crop), you can produce food in great quantity and be able to feed closer to 3-4 dwarves per tile of farmland.
Growing non plump-helmet crops for variety in booze (sweet pods and cave wheat) will reduce your booze-yield a bit since they don't grow quite as fast as plump helmets (pig tails do though), but is totally worth it for the happiness factor. Above-ground crops also tend to grow quite quickly, and the Berries(prickle, fisher, straw and sun) have the same "Food or Booze" property as Plump Helmets, and can similarly be grown year 'round. This can add to booze variety without impacting the availability snack food for impatient hungry dwarves.
If you are facing a sudden severe shortage in food, have no animals for slaughter, and can't wait for crops to grow, keep in mind that SEEDS ARE COOKABLE. I've warded off famine before by cooking up a load of "Plump Helmet Spawn Biscuits". The fact that seeds are cookable, means that the actual food yield of a crop is actually a little less then double the nominal value, depending on stack size. A size 3 stack of plump helmets contains 5 units of food: 3 from the mushroom itself, and 2 from the surplus seeds (one seed needs to get replanted). Seed-cooking is generally avoided since seeds have a very low value, and don't produce large stacks of cooked food.
Booze-cooking on the other hand is quite the value multiplier. 1 food = 5 booze, but if the booze is used in cooking, then all 5 units become food again. Just like with Quarry Bushes, you are quintupling your food yield. The large stack sizes of quarry bushes and Booze mean that they make for great companions for producing vast quantities of large-stacked food. (I personally consider this to be a bit of an exploit, and neither cook booze, nor use quarry bush leaves. This is because I feel that farming is too overpowered in DF.)
Using all these tricks, you shouldn't have any problem feeding your whole fort on only 50 or so tiles of farmland. Hunting wild animals, slaughtering livestock, milking animals to make cheese, or collecting eggs from birds will further reduce the amount of farmland required to support your population.
Due to all these many variables, achieving a balance between food consumption and production is quite tricky, and is generally done by trial and error, but then a herd of Camels wanders into the traps at your entrance, or your hunter takes down an Elephant and you're suddenly 200 food richer, totally wrecking your beautifully crafted "Balance".
Alternately, you could just embrace your massive agricultural surplus and use that as your fort's trade good. Sell off your excess while keeping your basic food reserve.
At the start though when your farmer's suck and you'd actually rather like to build up a surplus, just aim for 1 farm-tile per Dwarf. That'll do you just fine.