I'd think that one would want a wal_keep_size to cover the pending updates while the standby server might be unavailable, however long one might anticipate that would be.
In my case, I get a complete replacement (in the form of "|"- delimited ASCII files) of one of the SCHEMAs every Sunday. The size of that ASCII data is about 2GB, so I'm thinking of doubling that to 4GB (256 WAL files) to protect me in the case of the standby being unavailable during the update. Note that a complete loss of both servers is not catastrophic (I have backups); it would just be annoying.