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Try this:

warpPerspective(image1, result,homographyReln, cv::Size(image1.cols+image2.cols, image1.rows));

Try this:this, I think it should sort out your height, width requirements.

warpPerspective(image1, result,homographyReln, resultImage ,homographyReln, cv::Size(image1.cols+image2.cols, image1.rows));