We ran over one near our deer lease in Southeastern Oklahoma last week and it was probably 6'!!!!!!!
I seriously doubt it was a copperhead, if that's what you're saying. Most people misidentify rat/bull/chicken snakes and all their variations for copperheads. You'll rarely ever see a copperhead longer than 3 feet. In fact, 3 feet is a huge copperhead. This snake doesn't at all look like a copperhead. A copperhead's color can be anything from tan to almost red to dark brown. Sometimes they have very pronounced striping and sometimes they appear to be almost a solid color. Some have an iridescence bluish color to them. Instead of color, pay attention to a heart/triangle shaped head and a blunt tail.
As far as a cottonmouth is concerned, you're usually looking at a diamondback water snake, the most common water snake in Texas. You can usually smell them before you see them, unless one is just swimming by I guess.
And as far as killing them, the more non-poisonous beneficial snakes you kill the more non-beneficial poisonous snakes you're going to have.