Well there are about a brazillion variables that can affect the answer. Please be forewarned I have absolutely NO practical experience with this but we've been thinking along the same lines here. These are things we're considering:
How many people do you want to feed?
What types of grains do they like to eat (wheat, oats, corn, amaranth, barley...)?
How many animals?
What types of grain?
Is your climate suited to growing the grains you desire?
Once you've figured all that out, the amount of land required will vary depending on your soil.
This info is from John Jeavons' "How to Grow More Vegetables" book and will give you at least a clue
Wheat - 151 pounds consumed per person per year on average. On average you can get about 5 lbs. per 100 sq ft (no acreage figure available in this book).
Oats - 6.5 lbs per year, 4.4 lbs per 100 sq ft
Rice - 21 lbs per year, 13 lbs per 100 sq ft
Corn - 152 lbs per year, 17 lbs per 100 sq ft
Hope that's at least a jumping off point for you.