Start off with about 50GB of storage since this can easily be changed when dealing with VMs. 20GB is the initial storage but the additional GB of storage will be needed for your 1000 flows/s. There is no exact answer how much flows all your devices will have but there will be a better understanding once it's up and running. Then you'll be able to monitor your network and analyze the data collected and from that you'll be able to make a better judgement on how much storage more storage may be needed.
Just in the interest of clarity, netflow does not use a SQL database anymore and hasn't in several years, it uses a database technology called fastbit.
50 GB should be a good starting estimate for 100 devices to hold 30 days, even with "above average" utilization. The Netflow data compresses super effectively and the majority of clients I work with end up with flow databases in the 10-20 gb range