Adjective endings - http://www.nthuleen.com/teach/grammar/adjektivendungenexpl.html
Unser should immediately tell you that Teller is singular. Its adjective needs to show that it is a masculine nominative inflection, so it adds a 'er' to groß. "Unsere Teller" would be "our plates". With that said, Unsere großen Teller would be "our big plates".
That website really helped me out with this kind of stuff.