Different size drives for Raid 1 in Mirror?

I need to replace a failed drive for My Cloud Mirror Gen 2. Current drives are 2TB each.
Is it possible to replace the failed drive with a 4TB drive and rebuild a Raid 1? The thinking is to replace the other 2TB with another 4TB later on, and therefore expand capacity in the Raid 1 build.

Many thanks in advance.

Theoretically; yes.

Make a backup to another drive before you do any of this.