MongoDB Storage size scaling
You can more or less estimate that but you cannot be 100% sure.

A piece of advice I can give you is to change your storage engine to wiredTiger. I had 20.0GB of disk used by a mongodb instance using the MMAPv2 (which was the default storage engine before Mongodb 3.2). When I converted that to use wiredTiger my disk space dropped to unbelievable 1.9GB.

That might help you for a while.