Other correct answer: The tree isn't small, but is big
I don't agree. Translate for your sentence is "Ten strom není malý, ale JE velký."
That would translate "The tree is not small, but it is big"
Just use the report function for other correct answers ;-)
You mostly have 'ten' as that. Why is it The?
We have both the and that for ten throughout the course consistently.
Why not "this tree"
There is a difference between "this" and "the/that". For "this" (as adjective) we can use "tento / tato / toto / tenhle / tahle / tohle".