Unfortunatelly is imposible for me to get the correct measurements of the paining from the picture. What you have to do is use a ruler to measure the sides of the rectuangular painting.
If the length of the sides are 1/8 of the original size of the painting. To the measurements you do you have to multiply them by 8 to reach the original dimensions of the painting.
For example, if the cards dimensios are 5 x 7, then the original pinting dimensions would be
5*8= 40cm
7*8= 56cm
As I mentioned before, I cannot measure the card from my computer screen but that is the procedure you should follow to reach the correct answer.