I recently took some photographs of a Kestrel on the same perch and from the same position that I had photographed Peregrines previously.  I compared the images to get a good idea of the size differences between different species and more dramatically, the difference in size between adult Peregrines.  I have to confess to being staggered at the difference between the female and male Peregrines. To get a good accurate idea of the sizes you need to check the branch which in all pictures is the same size.