The Metalogger machine
A Metalogger on its own needs little from a machine. The Metalogger receives the Master Server's metadata over one connection. The connection carries a changelog stream plus a periodic full image. The Metalogger writes what it receives to disk and holds none of it in memory. The machine therefore needs disk space of the same order as the Master Server's, any POSIX-compliant operating system, and little else. That space is estimated with the Master Server's formula. Use the number of copies that this machine keeps and the image size of the Master Server, because the images on this disk are the Master Server's. The default number of images kept is higher here than on the Master Server, so the default requirement is the larger of the two. The Metalogger stream covers what is sent over that connection.
The plan to start a Master Server sizes the machine
No command turns a Metalogger into a Master Server. The machine has an advantage instead: it already holds a current copy of the metadata. When the hardware of the Master Server fails, install the Master Server packages on the machine of the Metalogger. Start the process there, on the metadata that is already on that machine. In the Community Edition, nothing takes the place of the Master Server automatically. In that edition, this is the recommended way to get a new Master Server running quickly.
Choose a machine planned for that job as a Master Server machine, not as a Metalogger. That machine needs at least as much RAM as the Master Server. Once the machine runs as a Master Server, the whole metadata set has to fit in the memory of that machine. Master Server RAM from the object count calculates that figure from the object count. The machine also needs a CPU of the same class, because a Master Server serves all of its clients from a single loop. Put the data directory of the machine on a redundant array, as on any Master Server machine. A disk failure there costs you the copy exactly when the Master Server is lost.
A machine that is only ever going to keep a spare copy of the metadata needs none of this. The requirement comes from the plan to start a Master Server on that machine, not from the Metalogger.