I think you have missunderstod BTRFS raid1.
You can mix different size disks and expand 1 disk at the time.
BTRFS raid1 is not like traditional raid1 that you find in for example ZFS, hardware raid or linux software raid (MD).
BTRFS will just make sure you have 1 extra copy of the data.
I use 1x3TB and 2x2TB disks in a single btrfs raid1. It works great for me. Later on i will switch out one of the 2TB disks for a 4TB disk.