Unfortunately, this won't do what you want. The larger disks will be "rightsized" down to the size of the original smaller disks in the plex. You'll end up with something similar to wht is shown below:
filer> aggr status -r aggr1
Aggregate aggr1 (online, raid4, reconstruct) (block checksums)
Plex /aggr1/plex0 (online, normal, active)
RAID group /aggr1/plex0/rg0 (reconstruction 1% completed)
RAID Disk Device HA SHELF BAY CHAN Pool Type RPM Used (MB/blks) Phys (MB/blks)
------------ --------- ---- -------- ------ -------- ------ ------ -------- ------------------ ----------------
parity 0a.19 0a 1 3 FC:A - FCAL 10000 68000/139264000 69536/142410400
data 0c.49 0c 3 1 FC:A - FCAL 10000 68000/139264000 137104/280790184