How do I change it's size?
openwrt version is 19.07
*This firmware runs on the VM X86-64 device
add something compile again
The /dev/root directory is still 100% occupied
add ca-bundle ca-certificates like this files ( kmod-fs-ext4 kmod-fs-exfat kmod-fs-vfat)
and use openwrt-x86-64-combined-ext4.img.gz
it's succeed now!
but still don't know! why can't use openwrt-x86-64-combined-squashfs.img.gz
I study by myself ! it's so so hard ! 
I hope someone can reply to me thanks
You forgot to tell us, which router you are talking about.
Most (old) routers have really limited flash space, so there might not be much possibilities for enlargement.
/dev/loop0 looks strange. some x86 device? Then you might have more possibilities.
Yes, it runs on VM devices!
Thank you for your answer.
Openwrt uses OverlayFS file system based on Overlay technology to combine the original read-only rootFs (SqushFS) partition and writable rootfs_Data (JFFS2) partition into one logical partition and mount it to the root directory
