For that size, you might do weekly L0 and daily L1 (differential) backups. Try to do L1 (cumulative) backups if possible.
Getting the total sizes of Segments (from DBA_SEGMENTS) will give you an approximation for the minimum size. (Tables/Indexes that have been dropped and are no longer in DBA_SEGMENTS wouldn't appear but the underlying blocks for them if unused by other Tables/Indexes would still be backed up as they'd have been formatted).
You could run the Backup as a COMPRESSED BACKUPSET to reduce the size.
Whether you want ASM or FileSystem depends on your comfort level. If you run backups to ASM, you can only use RMAN to backup from ASM to Tape. However, backups on FileSystem can be copied to tape using any other method (tape backup utility). Also consider if you want to replicate/copy the backups to other servers and what methods you'd have available.
Hemant K Chitale