"The bags are big and red."
Translation:Les sacs sont grands et rouges.
Unlike English, the plural is carried on to the adjectives just as the gender is. We are beautiful(s) - it's the same in Italian. The plural and gender affect adjectives and also possessives like "my, your". The object you're talking about pretty much controls everything in the sentence in terms of plural/singular and masculine/feminine
If DL suggested "valises" in response to your incorrect answer, I think it might be because you put "grandes" which is feminine. "valises" (plural of "valise") is also a feminine word that could be accepted as meaning "bags" ("suitcases").
"sacs" is masculine so the correct adjective form is "grands" (no 'e').
could be acceptable
is not grammatically correct.