In a nutshell, our MLS does not require it. We are required to put in the lot size (ie 100 x 120) and we can choose from a range of acreage categories (less than 1, 1-5, 5-10, 10+). The various websites such as trulia do not always recognize the field description so it doesn't get uploaded to their systems.
If you really want details, work with a local realtor who can set up a search with the parameters you want. You'll get updates on the properties that meet your acreage requirement.
I would be happy to do this for you.
Message me at firstname.lastname@example.org.