The phrase "than a foot" is an idiom that means "more than a foot." It is used to express that something is larger or greater than a foot in size, length, height or depth.
Sentences with «than a foot»
Of course, we got hit with more than a foot of snow days later. (isntthatcharming.com)
We raise our index finger in the air less than a foot away from the hand with the treat and he takes it ever so gently. (americanshihtzuclub.org)
Our faux tree collar's diameter was still a tad smaller than the feet of the tree stand. (stowandtellu.com)