PSNR of two images

i wanted to calculate the PSNR between input image and output image. I am using DWT for image enhancement i.e if the size of input image is 512*512 after applying all of my operation ill get a image of size 1024*1024(output image).If i want to calculate the PSNR b/w these images formula is as follows .

MSE =∑(i,j)(Iin(i,j)-Iorg(i,j))2/(M*N) PSNR = 10log10(R*R/MSE)

problem is: 1)Since the size of two images are different what is should be the value chosen for M and N. 2)while calculating MSE , we can get the value for :output(512,512)-input(512,512) what to for calculating : output(513*1)-input(513*1) and further more till 1024*1024, because input images size is 512*512.