DF2012:How large a farm do i need
Dwarves require approximately 2 units of food and 4 units of drink each season. Farming can produce enough crops to satisfy part or all of these requirements. However, be aware that the more and larger your farms are, the more time and effort must be accorded to their maintenance.
 The math
Your dwarves can plant 1 seed per tile on each farm plot, and depending on the crop, the skill of the grower, and whether the farm plot was fertilized, each planted tile will yield a stack of 0-12 (average: 5) edible, millable, brewable, or otherwise processable plants each harvest cycle. Brewing always quintuples stack sizes; for example, a stack of cave wheats  is brewed into a barrel of Dwarven beer  at a still. Processing quarry bush plants into quarry bush leaves at a farmer's workshop also quintuples stack size, as does processing sweet pods into Dwarven syrup. Milling sweet pods into Dwarven sugar does not increase stack size. Pig tails and plump helmets grow faster than other crops, and as long as the mature plants are harvested promptly, it is not hard to grow multiple crops of either plant each season from the same plot.
|Plant Name||Active Seasons||Days Per Harvest||Harvests Per Tile Per Season||Harvests Per Tile Per Year||Average Plants Harvested Per Tile Per Year||Average Processed Food/Drink Per Tile Per Year|
A beginning fortress has 7 dwarves, each of which consumes 6 urists of food and drink per dwarf per season, or 168 for the whole fortress for the year. The starting group can theoretically be fed by a single farm tile, but in practice they will need more because they are unable to use the farm tile to its fullest potential. A properly-managed and fully-utilized 3x3 plot could produce an average of up to 2700 units of food and drink per year, enough to feed an entire mature fortress. A single 5x5 plot could produce an average of up to 7500 units of food and drink per year, enough to feed over three hundred dwarves.
The general limitation is not the size of the farm per se, but the skill of the growers, the availability of seeds and potash for fertilizer, and careful management of labor, stockpiles, and the food industry to ensure that crops are harvested (rather than left to wither in the field), are properly stored away from vermin, the plants are processed, and seeds are recovered so that the cycle can continue next season.
Many fortresses begin with a single large farm (5x5) with plump helmets, or several smaller farms (3x3 or 1x5 are common) with a wider variety of crops. Given the small number of dwarves, a single smaller farm (2x4 or 3x3) is probably best for beginning fortresses. As the growers level up and farm yields increase the farm can be expanded and newer, smaller farms with different crops can be added to enable new growers to practice their skills. The more tiles of farm plot you have, the more growers (and harvesters) you will need.
To minimize the cost of fertilization, farm plots should ideally be one less than a multiple of 4 tiles -- ex. (1x3), (1x7), (3x5), (3x9), (5x7), (7x9)