You absolutely can. Of course you'll only be able to use as much capacity as the smallest disk. Sometime ago I was running a secondary mirror with one 8TB disk and 3 disks pretending to be the other 8TB disk. They were 4TB, 3TB and a 1TB - trivial with LVM. Worked without a hitch for a few years till I replaced the three gnomes in a trench coat with another 8TB disk. Obviously that's suboptimal but it works fine under certain loads.