Dogs need energy for growth, work, and lactation and so on, and obtain it by "burning up" (digesting) protein, carbohydrates, and fat. The energy content of food is measured in calories and while most people think of calories as a bad thing, they are only bad if you consume more than you need.
A dog's energy requirements are determined by its weight, and activity level. The smaller the dog the fewer calories are required. The following chart can be used as a good starting point.

Dogs Weight Energy needed (calories per day)

Small 5 lbs. 290
10 lbs. 410
20 lbs. 700

Medium30 lbs. 920
50 lbs. 1300
60 lbs. 1600

Large 70 lbs. 1810
80 lbs. 1950
90 lbs. 2190

These values should be increased for the following circumstances:
Growing puppies x 2
Pregnancy x 1.5
Lactation x 3

Based on the analysis of pre-packed dog foods, dry, semi-moist, canned, and biscuit, the following table gives a good idea of the number of calories per ounce of each type.
Dry 96.4 calories per ounce
Semi-moist85 calories per ounce
Canned28.4 calories per ounce
Biscuit 99.2 calories per ounce

Based on this information, if you have a 20 lb. dog, it will require 700 calories per day. To meet this requirement you will have to feed him approximately:
Dry 700 / 96.4 calories = 7.26 ounces
Semi-moist700 / 85 calories = 8.23 ounces
Canned700 / 28.4 calories = 24.64 ounces
Biscuit 700 / 99.2 calories = 7.05 ounces

Individual products may vary in calorie content, but most manufacturers will supply analysis on request.