Choose your county
Requirements are set county by county within a common state framework, and the practical answer differs more than most landowners expect: what documents well on a Bandera canyon tract is not what documents well on Llano granite country.
The rule everyone gets wrong
There is no minimum acreage for a tract that has not changed size since the previous tax year. The minimum only applies to tracts that have been reduced in size, typically by subdivision, and generally runs between 12 and 50 acres rising from east to west. If your boundary has not moved, acreage is not your obstacle.
What matters instead is that your land already holds an agricultural or timber valuation, and that you perform and document at least three of the seven qualifying activities each year. Start with the wildlife exemption calculator to size those activities to your acreage.