BzF:

This device has 16 MB flash, but with fresh install of recent LEDE
//lede-17.01.2-ramips-mt7628-miwifi-nano-squashfs-sysupgrade.bin
Reboot (17.01.2, r3435-65eec8bd5f)
//
on Xiaomi Mi WiFi Nano / Youth (R1CL) only 4 MB are available in /overlay:

root@LEDE:~# df -h
Filesystem Size Used Available Use% Mounted on
/dev/root 2.3M 2.3M 0 100% /rom
tmpfs 29.8M 60.0K 29.8M 0% /tmp
tmpfs 29.8M 48.0K 29.8M 0% /tmp/root
tmpfs 512.0K 0 512.0K 0% /dev
/dev/mtdblock6 4.3M 276.0K 4.0M 6% /overlay
overlayfs:/overlay 4.3M 276.0K 4.0M 6% /


With fresh instal of PandoraBox
//PandoraBox-ralink-mt7628-xiaomi-r1cl-squashfs-sysupgrade-r1468-20151001.bin
PandoraBox 14.09 r1468 / LuCI 0.12 (0.12+git-4d0a20b)
//
is available 10 MB:

[root@PandoraBox:/root]# df -h
Filesystem Size Used Available Use% Mounted on
rootfs 10.6M 428.0K 10.2M 4% /
/dev/root 4.0M 4.0M 0 100% /rom
tmpfs 30.1M 144.0K 29.9M 0% /tmp
/dev/mtdblock7 10.6M 428.0K 10.2M 4% /overlay
overlayfs:/overlay 10.6M 428.0K 10.2M 4% /
tmpfs 512.0K 0 512.0K 0% /dev