足の画像上で横幅を実寸で表すには

現在Pythonを勉強中の初学者になります。
足の画像上で2点を結び、その長さを測りたいと思っていたのですが
ユークリッド距離を使用したもののこれからどう実寸の距離を出せばよろしいでしょうか？

import cv2
import numpy as np
from matplotlib import pyplot as plt
from __future__ import print_function
from skimage.feature import peak_local_max
from skimage.morphology import watershed
from scipy import ndimage
from scipy.spatial import distance

img = cv2.imread(＊＊＊＊＊＊＊＊＊, 1)
height = img.shape[0]
width = img.shape[1]
img1 = cv2.resize(img , (int(width*0.5), int(height*0.5)))

class PointList():
    def __init__(self, npoints):
        self.npoints = npoints
        self.ptlist = np.empty((npoints, 2), dtype=int)#2次元配列
        self.pos = 0
        
    
    def add(self, x, y):
        if self.pos < self.npoints:
            f=self.ptlist[ self.pos,:] = [x, y]
            print("f",f)
            self.pos += 1
            return True
        return False
    


def onMouse(event, x, y, flag, params):
    wname, img, ptlist = params
    if event == cv2.EVENT_MOUSEMOVE:  # マウスが移動したときにx線とy線を更新する
        img2 = np.copy(img)
        h, w = img2.shape[0], img2.shape[1] #縦、横
        cv2.line(img2, (x, 0), (x, h - 1), (255, 0, 0))
        cv2.line(img2, (0, y), (w - 1, y), (255, 0, 0))
        cv2.imshow(wname, img2)

    if event == cv2.EVENT_LBUTTONDOWN:  # レフトボタンをクリックしたとき、ptlist配列にx,y座標を格納する
        if ptlist.add(x, y):
            print('[%d] ( %d, %d )' % (ptlist.pos - 1, x, y))
            cv2.circle(img1, (x, y), 3, (0, 255, 0), 3)
            cv2.imshow(wname, img1)
        else:
            print('All points have selected.  Press ESC-key.')
        if(ptlist.pos == ptlist.npoints):
            print("ptlist.ptlist",ptlist.ptlist)
            xx=cv2.line(img, (ptlist.ptlist[0][0], ptlist.ptlist[0][1]),
                     (ptlist.ptlist[1][0], ptlist.ptlist[1][1]), (0, 255, 0), 3)

            
            aa=np.array([ptlist.ptlist[0][0]])
            bb=np.array([ptlist.ptlist[0][1]])
            cc=np.array([ptlist.ptlist[1][0]])
            dd=np.array([ptlist.ptlist[1][1]])
            
            for a in aa:
                print(a)
            for b in bb:
                print(b)
            for c in cc:
                print(c)
            for d in dd:
                print(d)
                
            aca=np.array([a,b])
            bcb=np.array([c,d])
            df=bcb - aca
            print(np.linalg.norm(df))
            
    

if __name__ == '__main__':

    wname = "MouseEven"
    cv2.namedWindow(wname)
    npoints = 2
    ptlist = PointList(npoints)
    cv2.setMouseCallback(wname, onMouse, [wname, img1, ptlist])
    cv2.waitKey()
    cv2.destroyAllWindows()