ok, so I was not able to accomplish this task from inside my phone, for some reason modified filesystem's size shrinks back to 2GB even tho I run resize2fs few times. Partition was fine tho, it just FS.

For those who's trying to solve this issue:
Was able to change the file system size from a host, connecting the phone to my laptop and running

adb shell  # phone in recovery mode
e2fsck -f -y system.img
resize2fs -fp system.img

Hi,

Please help me, I'm confused...

So I'm trying to increase my root, did following:

dd if=/dev/null of=/userdata/ubuntu.img bs=1M seek=6144 count=0
e2fsck -p /userdata/ubuntu.img
resize2fs /userdata/ubuntu.img

got this

root@ubuntu-phablet:~# ls -lahi /userdata/
total 3.9G
      2 drwxrwx--x.  5 system      system           4.0K Jul  1 12:08 .
      2 drwxr-xr-x  23 lxc-dnsmasq systemd-timesync 4.0K Apr 30 19:54 ..
1310721 drwxrwx--x  23 system      system           4.0K Jul  1 13:21 android-data
     22 -rw-r--r--   1 root        root                2 Oct 13  1970 .last_ubuntu-build
     21 -rw-rw-rw-   1 root        root                0 Oct 13  1970 .last_update
     13 -rw-------.  1 root        root                2 Jan  7  1970 .layout_version
     11 -rw-------   1 root        root              32M Oct 13  1970 SWAP.img
 131073 drwxr-xr-x   5 root        root             4.0K Oct 13  1970 system-data
     19 -rw-------   2 root        root             6.0G Oct 14  1970 system.img
     19 -rw-------   2 root        root             6.0G Oct 14  1970 ubuntu.img
 655361 drwxr-xr-x   3 root        root             4.0K Oct 13  1970 user-data

root@ubuntu-phablet:/userdata# parted ubuntu.img
GNU Parted 3.2
Using /userdata/ubuntu.img
Welcome to GNU Parted! Type 'help' to view a list of commands.
(parted) print
Model:  (file)
Disk /userdata/ubuntu.img: 6442MB
Sector size (logical/physical): 512B/512B
Partition Table: loop
Disk Flags:

Number  Start  End     Size    File system  Flags
 1      0.00B  6442MB  6442MB  ext2

already did reboot couple times and still getting

root@ubuntu-phablet:~# df -h /
Filesystem      Size  Used Avail Use% Mounted on
/dev/loop0      2.0G  1.8G   82M  96% /

my specs:
nexus 5
Ubuntu 16.04.6 LTS

p.s.
All Things Ubuntu Library does not work for me

root@ubuntu-phablet:~# ./ATU_arm-linux
./ATU_arm-linux: error while loading shared libraries: libgdk-x11-2.0.so.0: cannot open shared object file: No such file or directory
root@ubuntu-phablet:~# ldd ./ATU_arm-linux
	libpthread.so.0 => /lib/arm-linux-gnueabihf/libpthread.so.0 (0xb6f01000)
	libdl.so.2 => /lib/arm-linux-gnueabihf/libdl.so.2 (0xb6eee000)
	libgdk-x11-2.0.so.0 => not found
	libX11.so.6 => /usr/lib/arm-linux-gnueabihf/libX11.so.6 (0xb6dfd000)
	libgdk_pixbuf-2.0.so.0 => /usr/lib/arm-linux-gnueabihf/libgdk_pixbuf-2.0.so.0 (0xb6dd4000)
	libgtk-x11-2.0.so.0 => not found
	libgobject-2.0.so.0 => /usr/lib/arm-linux-gnueabihf/libgobject-2.0.so.0 (0xb6d8b000)
	libglib-2.0.so.0 => /lib/arm-linux-gnueabihf/libglib-2.0.so.0 (0xb6cb3000)
	libgthread-2.0.so.0 => /usr/lib/arm-linux-gnueabihf/libgthread-2.0.so.0 (0xb6ca1000)
	libgmodule-2.0.so.0 => /usr/lib/arm-linux-gnueabihf/libgmodule-2.0.so.0 (0xb6c8e000)
	libpango-1.0.so.0 => /usr/lib/arm-linux-gnueabihf/libpango-1.0.so.0 (0xb6c48000)
	libcairo.so.2 => /usr/lib/arm-linux-gnueabihf/libcairo.so.2 (0xb6b90000)
	libatk-1.0.so.0 => /usr/lib/arm-linux-gnueabihf/libatk-1.0.so.0 (0xb6b69000)
	libc.so.6 => /lib/arm-linux-gnueabihf/libc.so.6 (0xb6a7d000)
	/lib/ld-linux-armhf.so.3 (0xb6f38000)
	libxcb.so.1 => /usr/lib/arm-linux-gnueabihf/libxcb.so.1 (0xb6a59000)
	libgio-2.0.so.0 => /usr/lib/arm-linux-gnueabihf/libgio-2.0.so.0 (0xb6956000)
	libm.so.6 => /lib/arm-linux-gnueabihf/libm.so.6 (0xb68de000)
	libffi.so.6 => /usr/lib/arm-linux-gnueabihf/libffi.so.6 (0xb68c8000)
	libpcre.so.3 => /lib/arm-linux-gnueabihf/libpcre.so.3 (0xb686b000)
	libthai.so.0 => /usr/lib/arm-linux-gnueabihf/libthai.so.0 (0xb6855000)
	libpixman-1.so.0 => /usr/lib/arm-linux-gnueabihf/libpixman-1.so.0 (0xb67cb000)
	libfontconfig.so.1 => /usr/lib/arm-linux-gnueabihf/libfontconfig.so.1 (0xb678e000)
	libfreetype.so.6 => /usr/lib/arm-linux-gnueabihf/libfreetype.so.6 (0xb6717000)
	libpng12.so.0 => /lib/arm-linux-gnueabihf/libpng12.so.0 (0xb66ef000)
	libxcb-shm.so.0 => /usr/lib/arm-linux-gnueabihf/libxcb-shm.so.0 (0xb66dc000)
	libxcb-render.so.0 => /usr/lib/arm-linux-gnueabihf/libxcb-render.so.0 (0xb66c4000)
	libXrender.so.1 => /usr/lib/arm-linux-gnueabihf/libXrender.so.1 (0xb66ad000)
	libXext.so.6 => /usr/lib/arm-linux-gnueabihf/libXext.so.6 (0xb6692000)
	libz.so.1 => /lib/arm-linux-gnueabihf/libz.so.1 (0xb6670000)
	librt.so.1 => /lib/arm-linux-gnueabihf/librt.so.1 (0xb665a000)
	libXau.so.6 => /usr/lib/arm-linux-gnueabihf/libXau.so.6 (0xb664e000)
	libXdmcp.so.6 => /usr/lib/arm-linux-gnueabihf/libXdmcp.so.6 (0xb663a000)
	libselinux.so.1 => /lib/arm-linux-gnueabihf/libselinux.so.1 (0xb6615000)
	libresolv.so.2 => /lib/arm-linux-gnueabihf/libresolv.so.2 (0xb65f5000)
	libgcc_s.so.1 => /lib/arm-linux-gnueabihf/libgcc_s.so.1 (0xb65cd000)
	libdatrie.so.1 => /usr/lib/arm-linux-gnueabihf/libdatrie.so.1 (0xb65b8000)
	libexpat.so.1 => /lib/arm-linux-gnueabihf/libexpat.so.1 (0xb6590000)