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If m1, m2 are your original images and you reduce size :

 resize(m1, m1r, Size(),0.5,0.5);
 resize(m2, m2r, Size(),0.5,0.5);

src are points in m1r and dst in m2r then you have to change last column of h

Mat h=findHomography(src,dst);
h.at<double>(0,2) *=2;
h.at<double>(1,2) *=2;

you can check that it's OK with

warpPerspective(m1, m3, h, Size(m1.cols,m1.rows));

m3 must be like m2