There shouldn't be a comma between large and old as per English rules. Not a big deal, but since this course is pretty awesome, might as well make it perfect.
Can I say «Suuri vanha kuusi on metsässä»?
That would translate more like “the large old spruce is in the forest”, the focus would be on the spruce, while here the focus is on the forest, which has that random large old spruce.
Why does it matter if we put suuri or vanha first?
It shifts the focus in the description of the spruce. But for the purpose of DL- you are translating, so if you switch them, the app sees it as if you are saying that vanha is large and suuri is old.