Well, Having been in France and to the Louvre museum and seen the actual Mona Lisa, the one leonardo pulls out is very close in size to the actual painting. Yes true it is on wood as said in another post and is not a very big painting, but then again Leonardo didn't pull out a very big painting. It's very close in size to the real one.
I have researched the validity of this claim and here are my findings:
The dimensions of the Mona Lisa are 77 x 53 centimeters, which is 30 x 20 7/8 inches. So, the size of the painting in the film is very close to the size of the actual painting. Case dismissed.