They have a big dinner 和 They have a different dinner都正确,只是表达的意思不同.
They have a big dinner 意思是：“他们吃大餐”.
They have a different dinner意思是：“他们吃不同的饭”
He does something different 和 He does something big表达也是正确的.【做定语修饰sth的形容词都要后置】.