> +             return MAX(DEV_SIZE_MB_BTRFS, DEV_SIZE_MB_DEFAULT);

I do not think that we should harcode this here. I would be for a more
dynamic approach, i.e. add a function into the tst_supported_fs_types.c
that loops over supported filesystems and chooses max over the minimal
values for all supported filesystems. That way if we run on embedded
targets the device size will be 16MB as long as btrfs-progs is not
installed. Also that way we can easily define minimal size for xfs 300MB
and things will work for embedded as long as xfs-progs are not
installed.