If it is just an estimate then it would be around 4. 15.1 is pretty close to 16 which has a perfect square root of 4. Since it is a square both length and width would be the same. To check it you can just square 4 or 4 times 4 and get 16. Hopefully I helped! And sorry if I am wrong.
BTW the exact answer would be 3.88~ (square root of 15.1) just incase you needed that. :)