Ex

cv::Mat img, convert_img;
︙
//縦横どっちか長い方は？
int big_width = img.cols > img.rows ? img.cols : img.rows;
//割合
int width = 1000;
double ratio = ((double)width / (double)big_width);
cv::resize(img, convert_img, cv::Size(), ratio, ratio, cv::INTER_NEAREST);