On purple paint, I haven't looked it up but have unofficially heard that it may need to be 12 inches, top to bottom to be legal. I've seen a lot of guys just throw an old tyre over a fencepost, and paint that.
Unless you're working cattle or building rodeo chutes, lots of modern fence is just little metal T-posts, stretched with woven or barbed wire. Not many square inches to the post.
According to the internet
"According to Central Texas Geocachers, to act as a “No Trespassing” sign, purple paint markings in Texas “must be: vertical, at least 8 inches long, at least 1 inch wide. [The] bottom of the mark should be between 3-5 feet above the ground. Markings can be no more than 100 feet apart in timberland. Markings can be no more than 1,000 feet apart on open land, [and] they must be in a place visible by those approaching the property.”"