If you aren't bothered about cage, 28 disks total, 14 disks per node. So long as you stripe size is lower than the total disks per node then you're good to go.
If the stripe size isn't a a factor of 14 then dependent on the number of rows employed you have the potential to have a small amount of unused space (we're talking a few GB's of chunklets per node) where the array can't form the last stripe as there aren't enough remaining chunklets. See simplified example below, which represents chunklets on disk creating raidlets which form LD's and eventually volumes.
The smaller the stripe the less overhead for parity calculations and subsequent updates but you'll probably only see a difference if you are really pushing the array which won't be possible with so few disks. Obviously the wider the stripe the more efficient as there's less parity overhead 2+1(33%), 3+1(25%), 4+1, (20%), 5+1 (17%), 6+1(15%), 7+1(13%), 8+1 (12%). But the wider the stripe then mathematically, the higher the likelihood of a dual disk failure within that stripe.
So your optimal would be 6+1 (2x7x2nodes) on Raid 5 that way you can in theory fill every chunklet. Remember raid is built at a chunklet not a disk level so don't get too hung up about it, each drive can house multiple raid type and stripe sizes. That's why we have CPG's and unless you're thick provisioned you'll likely never fill them all anyway.
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