I've never heard of RAID 5\JBOD, but I would imagine that it uses the same to calculate parity storage as standard RAID5:
(N - 1) * Smin
N is the number of disks in your RAID set
S is the smallest disk in the RAID set. Obviously, you want all units the same, so use GB for large drives such as 1TB = 1000GB
So in this example, if you have
1TB, 500GB, 2TB, 750GB
Your formula would look like:
(4-1) * 500GB
Your total storage would simply be the sum of the disks so: 4.25TB
This means that the controller will carve out 1.5TB out of your 4.25TB RAID, leaving you with 2.75TB usable.
You should always be able to swap a disk as long as the replacement disk is equal to or greater than your existing disk.
So if your 500GB disk fails in the above example, you should be able to put in another 500GB or larger hard drive and the RAID will continue to function. However, this does not mean that the Virtual Disk will suddenly increase in size. Typically, although the replacement disk is larger, the controller will only use the space that was originally allocated when you setup the RAID set. In order to increase the size, you would have to rebuild the RAID.
With that being said, I've never heard of a combined RAID5\JBOD option!
Hope that helps!