"Your coat is long."
Translation:Dein Mantel ist lang.
It says coat, not jacket, but it wants you to translate to jacket, not coat.
What was the entire sentence that you typed?
Remember that Mantel is masculine so you need the form dein in front of it -- if you use deine Mantel, for example, with the feminine deine in front of it, that would be wrong.
"Deine" is used before feminine or plural nouns in the nominative and accusative case. "Mantel" is masculine (and in the nominative case here), so you have to use "dein". (Similar to how you can only say "der Mantel" and not "das Mantel" or "die Mantel")
The noun is Der Mantel so your would be dein. Deine appears for nouns with "Die"
Would "Ihr Mantel ist lang" mean the same thing but in a more polite/formal way?
Yes -- the word Mantel is masculine, but deine is either feminine or plural, so the two words don't fit together.
You should have written dein Mantel.
So only two possible options ihr and dein, but not euer because mantel is singular - one coat?
euer Mantel ist lang is also accepted.
Note that Mantel has to be capitalised.
Mantel is a noun and has to be capitalised
It's a masculine noun, so you would need euer Mantel for one coat belonging to several people at once.
eure would come before a feminine or plural noun, e.g. eure Mäntel "your coats".
So, do we ever qualify by saying "something is too long" as we do in English?