Sizing an in-memory database does not follow conventional database sizing rules.
For traditional databases, you buy a decent server machine, likely one with many CPU cores and reasonable memory, and then focus on application IOPS (I/O Operations per Second). If you are really going to stress the database, you must choose disks that can support the I/O needs of your application, today and in the future. Because these systems often use many disks to achieve high I/O performance, capacity is usually an afterthought.
With in-memory databases, throw out everything you know about sizing databases.