Hi Andy,
Your sizing questions really need to be answered by you as the answers will be based on the following aspects:
how big is your system?
how big are the tables you are planning on archiving?
how many years has the system been live?
how many years of data will be able to be archived based on residence time?
what is the annual growth of the tables that will be archived?
The answers to your questions about the execution of archiving have been told multiple times in this forum.
As for the hardware requirements, those questions will be answered by the ILM certified storage provider.
Best Regards,
Karin Tillotson