Note that while ombre is feminine, it starts with a vowel, which explains the use of "Son ombre" instead of "Sa ombre".
Ah, thank you! I was trying to figure out why grande was used instead of grand.
"grand" takes a "e" at the end when the noun it's related to is feminine.
Yes it does. Both should be accepted here, if it's not already the case.
The sound of "ombre" is wrong, it should have only 1 syllable.
Edit: Actually, 2 syllables is not wrong but sounds more like poetry.
Shouldn't this answer be...son ombre (au) mur est grande. The shadow is not sitting on top of the wall.