Not sure about Bexar County. Wilson County requires at least 10 acres if your property is in a platted subdivision. I could find no minimum acreage requirement for property outside of a platted subdivision.
State law prohibits counties from enacting certain rules on property over 10 acres. I'm paraphrasing severely here, but that's probably why you've heard that. And just shooting occasionally is quite a bit different than establishing a permanent range. You should just check your county's rules.
In another county where I want to find the answer to this question, the people at the County commissioners court were very helpful over the telephone. I would search for that information online and give them a call.
Timely post, as I just researched this very topic.
State Law says nothing about rural, unincorporated areas in the Counties.
County Courts, however, may regulate the discharge of firearms in unincorporated areas.
On November 21, 1990, the Bexar County Court issued an order that it is illegal to discharge a firearm on ten acres or less in an unincorporated area of the County.
So, if you own, control, or have permission from the land owner of 10.01 or more acres, you are good.
If nothing else, call your county commissioner and have him e-mail or fax you a copy of that order. Fold it up and keep it in your hip pocket if a deputy shows up because of neighborhood complaints.