The course contributors go through the reported possible alternative sentences and make individual decisions about each one.
In this instance, the course contributors may be making a point about the different ways English and Spanish puts possessives together. For that reason, I can imagine them not adding this particular alternative translation, even though it's technically correct.
MagosiaDoo2, I have never heard of such a rule nor could I find one. Please provide a link if you have proof otherwise as it is quite common to hear the phrase 'very large' in English.
The two words can be used interchangeably when modifying adjectives like the word 'large'.