I think I recall Jellei (who is extremely thorough in his explanations by the way), once saying somewhere in one of his comments about the whole....
"This is Y": entire phrase of Y takes nominative
"X is Y": entire phrase of Y takes instrumental.
In that comment, he said that if Y is lift only as an adjective, (ie. without a noun), the adjective will remain in nominative form.
But will still take the gender and quantity of X.
(I only started to learn polish a few weeks ago, so I may be wrong, but I vaguely remember him explaining something like that)
"dobry" is masculine, "dobra" is feminine. So is "zupa".
Copying my comment from under another one of your comments:
Gender. Every adjective has to match the gender of the noun/pronoun it describes.
"duże" is neuter (or 'not masculine-personal plural'). The word "dziecko" is neuter, so it "duże dziecko". As we mentioned 'not masculine-personal plural', "dzieci" (children) belong to this plural, so it's "duże dzieci".
"duża" is feminine. "duża książka" = "a big book".
"duży" is masculine. "duży kot" = "a big cat".
And finally there's 'masculine-personal plural', used for 'groups with at least one man'. "duzi ludzie" = "big people". This form is usually quite different from the other ones.