I was driving east through Austin Ranch. I was in the left lane because bikers are sometimes in the right lane.

On the ground wasn't a turtle with a black shell.
I quickly thought, if I see a spider leg--
Then I saw a big fuzzy spider leg.
It was a tarantula.

It was about the size of an adult hand over an adult fist.

http://www.amazingtarantulas.com/tarantulafacts.htm

How big can tarantulas get?

There are a few species of tarantula that will grow to be 9 or 10 inches, but the largest on record is the Theraphosa blondi (Goliath Birdeater), which can reach approximately one foot (12 inches) in length.