It’s three years since Apple last offered a Mac which could be configured with an internal RAID array, so there are a lot of Macs which now run external RAID systems. Browse their user documentation, and most seem very restrictive in terms of the drives which they can support: the Promise Pegasus R4 and R6 models, for instance, specified 1 and 2 TB units.
In fact many, if not most, external drive arrays can use – and make excellent use of – larger and faster drives than those original specified for them.
The first important question is to determine whether yours is a software or hardware RAID system. Hardware RAID systems have a hardware controller built into the unit, which you control through a utility; in the case of Promise Pegasus models, that is the Promise Utility. Some older hardware RAID controllers can be quite picky as to which drives they will support, or may not support drives larger than a certain size.
For software RAID, most modern units should be fairly agnostic as to what drives you use, as it is the software (AppleRAID or SoftRAID) which does the work. If you’re using large or fast drives, and are taking the issue seriously, you should opt for SoftRAID; note that you will need to use SoftRAID version 5.5.5 or later to run with macOS Sierra, as previous versions are not fully compatible.
Next, you must decide whether you are replacing one or two failed drives, or intend upgrading the whole array. Whether software or hardware, RAID systems generally work best and fastest when all the drives in the array are the same model and size. The further that you depart from that, the poorer the array will perform. There is no point in putting one 2 TB SSD in an array of 1 TB hard disks, and some RAID controllers may not work properly with such a mismatched array.
When replacing a failed drive, it is often hard to get an identical model; in those circumstances, try to get the closest match in terms of type (HDD/SSD), interface (SATA specification), spin speed, manufacturer, and capacity.
When upgrading an array, check the experience of other users, and compatibility listings available from the vendor. For example, Promise Pegasus R4 drives can safely reach 4 TB, according to Creation Video’s experience. They also discovered a list of drives which had been tested by Promise, and selected one of those.
Contacting the vendor’s support service can sometimes be helpful, but they are unlikely to provide any information beyond that of their own compatibility testing. Specialist storage suppliers may also be prepared to let you return a set of drives if they do not work: talk to them and see if you can agree this.
Assuming that you have chosen the model of drive with which to upgrade your RAID array, there is one final task: try to obtain each drive from a different manufacturing batch. This is tricky if you are also aiming to get the best price, as it may require you to buy from two or more different suppliers.
There is extensive experience showing that hard drives from the same batch tend to fail after about the same level of usage. If you put four or six drives from the same batch into a RAID array, which itself tends to balance use across each of the drives in the unit, then they are most likely to fail within the same relatively short period of time. Most RAID systems and levels are designed to cope well with single-drive failure, but the prospect of two or more drives in the same array failing within a few hours or days of one another should be a serious concern.
(Thanks to Jan for the links on Promise Pegasus RAID systems.)