btrfs: HUGE metadata allocated

First, make sure your kernel and the BTRFS user-space is up-to-date.

According to the BTRFS FAQ, you cannot resize the metadata block size. You can, however, try to run btrfs balance start -m <mount point> which will reallocate the metadata across the entire disk and, as a side effect, possibly free up unallocated space.

If that doesn't work, you may have to backup your data and recreate the filesystem. (Take a look at this for more info.)