How to size FRA or TPITR?

Maybe this is a two-part question:

Is there a formula or guidance for sizing the Fast Recovery Area? We want to use features like flashback recovery but are not sure how much space our database will need and can't risk it just blowing up and halting.

For 12c Table Point-In-Time Recovery, is there a way to estimate size for this as well? I have well over the needed space of my data files volume and am using Oracle's recovery method but continue to get the error:

PLS-00302: component 'SETSKIPOFFLINERANGEABOVESCN' must be declared

ORA-06550: line 1, column 7:

PL/SQL: Statement ignored

RMAN-05122: there is insufficient disk space to perform this table recovery