Can you advise how much time would Genesys would take to prepare a specific size file , for instance how much time for a 20 as opposed to a 200 MB file.
There is no realistic way for them to provide that answer. It would depend on too many factors, including the current processing load and other bandwidth constraints.
Most companies will not provide estimates that can be construed as promises outside of what is in their published SLA or negotiated contract. Genesys' is here.
There is not a specific answer to that question. Well-behaved apps will subscribe to the v2.users.{id}.outbound.contactlists.{id}.exportnotification topic prior to initiating the export to be notified when the export has completed.