It seems that you don't understand the meaning of the original sentence. Translate it into Chinese first: the library in Paris is very big.

Ask a question about "Paris", of course, "which one-Quelle".

Also, it would be better if you have a certain English foundation:

The library in Paris is very big. The problem in Paris should be like this:

Which library is big?

2/ If ou is used, the original sentence should be like this:

There is a big library in Paris.

-Where is the big library?