Carefully consider the work that goes in to keeping up with 5, 10, 15 etc acres of land. No good having a huge shed if all you have time for is mowing/gardening etc. IMO 2 acres is a maximum if you are not going to use the land for some sort of crop or herd.
On the other hand, 5 acres would lend itself nicely to three or four head of cattle or a dozen sheep, a few chooks a decent vegie patch and as big a shed as you could possibly want. The cattle/sheep would pay for themselves and keep your freezer full. Good fresh eggs and vegies...yum.
Dan