How to use instrinsic camera matrix after rectifying image.

After rectifying a pair of stereo images by rectifyStereoImages function, the size of output image pair has been changed. But my camera matrix is calibrated from the original image size. In this situation how do I use the original intrinsic camera matrix. e.g I need to back-project the 3D points reconstructed from the disparity map of the rectified images. Currently, the back-projection did not meet the desired pixel locations.