How Much Land?
In some cases, you can lease as little as one or two acres, if the farmer is nearby and will not have to transport his equipment. If your land borders other hay ground, chances are good the farmer who cultivates it will be happy to include your area. If a farmer would have to travel or transport equipment, you will probably need 10 acres or more to make it worth his while.
How Much Money?
In some cases, the farmer will buy the hay outright from you, or pay a monthly/yearly amount to lease the land. In other cases, you will be given a share of the hay once it is put up, and you can turn around and sell that hay to generate income. Figure out what is best for your situation and find a farmer who is willing to work with you.
To find someone to cultivate your land, you can ask at local feed or tractor supply stores, or post an ad in your local newspaper and on Craigslist.
Leasing your unused ground can be an easy source of passive income, but make sure you work out the details such as payment, lease term and other conditions before giving access to your land.