How to get image of specific size using python?

I am trying to get pictures from my camera using open cv I am using this code to access camera and save images in a folder.

import cv2
i = 0
#i = int(input(" "))
cap = cv2.VideoCapture(0)
while True:
    ret,frame = cap.read()
    frame = cv2.cvtColor(frame,0)
    cv2.imshow("frame",frame)
    if cv2.waitKey(1) & 0xFF == ord('q'):
        cv2.imwrite(r"C:\Users\naman.sharma\Desktop\Image_dataset\nm_result_{}.png".format(i), frame)
        break
cap.release()
cv2.destroyAllWindows()

This code works well but I am getting an image of size (600,480) which is very big for the project I am doing.

Is there any specific way to capture images with a certain size. I am trying to capture picture of (250,200).

Thanks for help in advance.