"My coat is large."
Translation:Mein Mantel ist groß.
German has three genders, masculine, feminine and neuter (der, die and das).
In the nominative case (the subject of the sentence, basically), you use mein for masculine and neuter words and meine for feminine words.
Der Mantel is masculine.
Adding to the explanation of Delta1212, you use "meine" for all nominative plural words (masculine, feminine, or neuter): meine Männer, meine Frauen, meine Kinder.
Why not, to translate it to: "Mein Mantel ist weit"? (I tried it, and it was considered wrong)...
“Weit“ is used to discribe distances in areas. “Der Junge hat den Ball weit geworfen“. “Lang“ refers to the object itselfe.
To my understanding, "large" may include the aspects of "long" and "wide". It can also refer to a (dress) size. "Groß" covers the same meanings, while "weit" is just "wide" (it does not say anything about mini-midi-maxi length, and it is not used for size classification XS-S-M-L-XL).
While "large" is often used in the sense of wide/weit, Duolingo seems to reject translations which may work in many cases, but not always.