When going to build a new RAID, you need three factors right.
1. Redundancy.
RAID does not replace a proper backup. You still have to back up your data somewhere in a safe place. RAID can not save you from operator errors or fire, earthquake, or whatever is common in your area. Anyway, most RAID types would still be operational when one of the array's drives fails. These include RAID1, RAID 1+0, RAID 4, RAID5, RAID 6, and exotics such as RAID 5E, RAID 5 EE, and Diagonal Parity RAID (RAID-DP).
2. User-accessible Capacity.
The capacity of the RAID array determined by the maximum drive capacity available, number of ports on the RAID controller, and the capacity required to maintain redundancy. Should you need simple capacity calculations done for you, use this free RAID Calculator.
3. Speed requirements.
Among the redundant array levels, RAID1 and RAID10 are the perferred choice on random writes.
If the array is almost always used read-only (e.g. a media library), or a write-once-read-never pattern (like a backup), then RAID5, RAID6, and variations of those are OK. If fast random writes are required, opt for RAID 1+0.
Bonus: A little different perspective of the RAID planning and requirements.
