In Python, how can I reduce a list of contours to those of a specified size?

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I am not sure if this would work for you but you can make use of contourArea method to get contour area and filter out some small contours on a basis of that. Here is the link.. Example code as below:

threshold_area = 10000     #threshold area 
contours, hierarchy = cv2.findContours(threshold,cv2.RETR_TREE,cv2.CHAIN_APPROX_NONE)   
for cnt in contours:        
    area = cv2.contourArea(cnt)         
    if area > threshold_area:                   
         #Put your code in here

Another elegant solution would be get a minimum area rect and get width and height from rect's 4 coordinates (by calculating distance on your own). But I'm not sure if you're using it to detect regular shaped contours and not.

And as far as I understand, you code should be rearranged as follow for it to work:

contours, hierarchy = cv2.findContours(im,cv2.RETR_TREE,cv2.CHAIN_APPROX_SIMPLE)
for cnt in contours
    rect = cv2.minAreaRect(cnt)       #I have used min Area rect for better result
    width = rect[1][0]
    height = rect[1][1]
    if (width<widthmax) and (height <heightmax) and (width >= widthMin) and (height > heightMin):
        #put your code here if desired contour is detected

Hope it helps.