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the main problem here is, that a writing video file needs a constant frame size, while size of your crops will vary due to person size & distance. so try following approach:

Size dstSize(64,128); // dst video size
VideoWriter vid("my.avi", VideoWriter::fourcc('M','J','P','G'), 10, dstSize); 

// later, in a loop, detect humans
Rect rec = ...
Mat cropped = frame(rec);
Mat dst;
resize(cropped, dst, dstSize);
vid.write(dst);