"Our big plate is green."
Translation:Unser großer Teller ist grün.
Here's my own take on it. I got rid of the notion of "weak" declensions simply by observing that the adjective is weak when the determiner is strong and the adjective is strong when the determiner is weak (i.e. doesn't have a dieser-word ending) or missing. It was easier for me, anyway, and more general too.
"Unser" is a deceptive word. It's an ein-word root -- all of it, including the final "er". So think what you would do if you had an ein there, and then substitute "unser" for "ein".
Here, you'd say "ein großer Teller." So it's "unser großer Teller."
(Note that großer is being a good adjective here, and telling us what the gender is with its nice -er masculine ending, because "ein" just isn't up to that job.)
If it was accusative, it would be "einen großen Teller" -> "unseren großen Teller"