So I entered "Meine Mantel ist groß" to which it marked wrong, but only the word "Mantel" was marked wrong as the error. The solution it proposed was "Meine Jacke ist groß", could anyone tell me why?
I can only guess: You used the female article ("Meine") and the DL routine 'concluded' that you were trying to use a female noun. "Mantel" is male, "Jacke" is female (and is a possible translation for a short coat).
Adding to the explanation of Delta1212, you use "meine" for all nominative plural words (masculine, feminine, or neuter): meine Männer, meine Frauen, meine Kinder.
English "large" is not the same as Spanish "largo". "Large coat" primarily refers to big size and wide cut of the coat, and only secondarily (or not at all) to length.