NTA uses the same database as Orion NPM. That is a fairly large interface so I would look for your database to grow about 40 - 50 gigs. This is a rough estimate. You can manage the database size by adjusting the compressed and non-compressed data retention times to values lower than the defaults. There are a couple of factors that are probably out of your control that will determine the ultimate database growth. These include the number of flows per second captured by the exporter (device) and the IP address range of endpoints in the flows (lots of addresses vs few addresses). If adjusting the data retention times does not control the growth, you can consider using sampled NetFlow on the exporter, which will summarize the flows before sending them to the collector (NTA).