Can’t be done. The sum of the array would also be the number of drives (n) multipied by the size of the SMALLEST drive in the array (s), So if you’re array had 2 X 1T and 2 X 2T drives, to all effects it will be just 4 x 1T.
I’m not sure what happens when you progressively replace and rebuild the array by replacing & rebuilding one 2T drive at a time until all were replaced.
But in any case, given the snail like performance of the WDSharespace, I suspect life is too short to be wasting it this way.
By the way, my understanding from a number of disaffected posts on the web is that it isn’t possible to rebuild RAID5 arrays on the Sharespace because the OS is striped across the array, But I could be wrong, I often am.