For larger parity raids, enterprise drives are best. HGST Ultrastar, Seagate Constellation or WD RE.
As to size, that depends on your storage needs. Provided the controller has the necessary ports, you can always expand the array by adding extra disks, getting more speed and more space.
Also have a look at Tweakers Page and subsequent articles, where Harm has added and expanded on previous articles.
I have eight 2tb WD RE drives in a raid 5 with one of them a hot spare. This I use for my day to day video needs. For backup I have a Drobo with eight 3tb WD RED drives, this is configured with two hot spares. The Drobo is not fast enough for online video work. At the end of each project I also backup to a removable HDD. The project files I 'save a copy' to the cloud and a copy of the camcorder files also go to a removable hdd. This may all seem over cautious but I had a near miss once when my motherboard raid setup failed.
@cc_merchant and Richard - thank you for your responses.
So can I start with 5x2TB WD RE disks (for media cache/previews/projects/media/exports) and set them up in a RAID or would I need even more disks?
I would use as many discs as you can afford, it is quite a hassle to add extra discs later. Most or my RE discs were bought off eBay some from here in the UK and some from the US. When I came to register them with WD I found there is a limit to the number of discs that they would register if bought outside your home country.
If you start with 5 x 2TB disks in raid 3 or 5, you will have a net space of (5 - 1) x 2TB = 8TB.
When you later add a 6-th or 7-th disk, by expanding the array, you will increase storage by 2 TB for each added disk.
At the same time you will increase the transfer speed of the array.
Expanding an array is not difficult, just time consuming, especially for a raid5, because all the data need to be rewritten to all the disks, but also the parity needs to be recalculated and written to all member disks because of the distributed parity in raid5. Raid3 has the advantage of writing parity only to a single dedicated disk.
I suggest you only use identical disks in the array. If you now buy 5 identical ones and at a later date decide to add more disks, make sure the new disks are identical to the ones you now use.