This is the 3250th most frequent Swedish word.
stor
Big; large.
Here, 'stor' (in the form 'stort' due to neuter singular agreement) describes the size of a tree.
Jag såg ett stort träd i parken.
I saw a large tree in the park.
In this case, 'stor' (unchanged due to common singular agreement) is used to describe the extent or size of a family without a physical reference.
Vi har en stor familj med många barn.
We have a large family with many children.
The adjective 'stor' again in neuter form 'stort' is used comparatively to emphasize size between objects.