How many contacts I can import in the system, and what should be the size of my list?

There is no restriction on the number of contact lists or number of the contacts in a list.

You can upload as many as contacts you want to since there is no limit on a number of contact you have.

But definitely system has its own limit and capacity to upload any list at a particular time.

Your list size should be less than 20 MB file for a specific file during a single upload, although you can split a bigger list into the smaller lists and upload them one by one.