After some more play here is what you need to know and do.
Ubuntu touch is booting from an android device so the partions have to be apropiate.
The android system is identifying the partitions by the partition label.
You have for meizu pro 5 this partitions: system (the root partition), cache (used for upgrates by Ubuntu, you should have about 1G on it), custom (no idea what is used for), userdata (used for /home and a lot of other folders that are mounted with bind to different parts of the root)
phablet@ubuntu-phablet:~$ sudo fdisk /dev/sda
Welcome to fdisk (util-linux 2.25.2).
Changes will remain in memory only, until you decide to write them.
Be careful before using the write command.
Command (m for help): x
Expert command (m for help): p
Disk /dev/sda: 58.2 GiB, 62537072640 bytes, 15267840 sectors
Units: sectors of 1 * 4096 = 4096 bytes
Sector size (logical/physical): 4096 bytes / 16384 bytes
I/O size (minimum/optimal): 16384 bytes / 8192 bytes
Disklabel type: gpt
Disk identifier: 00042021-0408-4601-9DCC-xxxxxxxxxxx
First LBA: 6
Last LBA: 15267834
Alternative LBA: 15267839
Partitions entries LBA: 2
Allocated partition entries: 128
Device Start End Sectors Type-UUID UUID Name Attrs
/dev/sda1 1024 1279 256 EBD0A0A2-B9E5-4433-87C0-xxxxxxxxxxxx D117F98E-6F2C-D04B-A5B2-xxxxxxxxxxxx private
/dev/sda2 1280 1343 64 EBD0A0A2-B9E5-4433-87C0-xxxxxxxxxxxx 25718777-D0AD-7443-9E60-xxxxxxxxxxxx proinfo
/dev/sda3 1344 1407 64 EBD0A0A2-B9E5-4433-87C0-xxxxxxxxxxxx 8A4BB8B4-E304-AE48-8536-xxxxxxxxxxxx misc
/dev/sda21 2048 3071 1024 EBD0A0A2-B9E5-4433-87C0-xxxxxxxxxxxx 08992135-13C6-084B-9322-xxxxxxxxxxxx param
/dev/sda22 3072 5119 2048 EBD0A0A2-B9E5-4433-87C0-xxxxxxxxxxxx 333A128E-D3E3-B94D-92F4-xxxxxxxxxxxx efs
/dev/sda23 5120 5631 512 EBD0A0A2-B9E5-4433-87C0-xxxxxxxxxxxx F20AA902-1C5D-294A-9177-xxxxxxxxxxxx pnv
/dev/sda24 5632 6655 1024 EBD0A0A2-B9E5-4433-87C0-xxxxxxxxxxxx DB88503D-34A5-3E41-836D-xxxxxxxxxxxx ldfw
/dev/sda25 6656 7679 1024 EBD0A0A2-B9E5-4433-87C0-xxxxxxxxxxxx 012B3303-34AC-284D-99B4-xxxxxxxxxxxx dtb
/dev/sda26 7680 13823 6144 EBD0A0A2-B9E5-4433-87C0-xxxxxxxxxxxx FAEC2ECF-8544-E241-B19D-xxxxxxxxxxxx bootimg
/dev/sda27 13824 22015 8192 EBD0A0A2-B9E5-4433-87C0-xxxxxxxxxxxx F13A0978-B1B5-1A4E-8821-xxxxxxxxxxxx recovery
/dev/sda28 22016 30207 8192 EBD0A0A2-B9E5-4433-87C0-xxxxxxxxxxxx B710EB04-45B9-E94A-8D0B-xxxxxxxxxxxx bootlogo
/dev/sda29 30208 35327 5120 EBD0A0A2-B9E5-4433-87C0-xxxxxxxxxxxx AD5EC4B6-2D9F-8544-9417-xxxxxxxxxxxx rstinfo
/dev/sda30 35328 40447 5120 EBD0A0A2-B9E5-4433-87C0-xxxxxxxxxxxx CCEB0B18-39CB-D547-9DB7-xxxxxxxxxxxx mnv
/dev/sda31 40448 45567 5120 EBD0A0A2-B9E5-4433-87C0-xxxxxxxxxxxx D4981A2B-0478-544E-9607-xxxxxxxxxxxx reserved1
/dev/sda32 45568 50687 5120 EBD0A0A2-B9E5-4433-87C0-xxxxxxxxxxxx 6D6C9A36-E919-264D-A9EE-xxxxxxxxxxxx reserved2
/dev/sda33 50688 55807 5120 EBD0A0A2-B9E5-4433-87C0-xxxxxxxxxxxx 60B98C0E-BEAD-B043-9CC6-xxxxxxxxxxxx reserved3
/dev/sda41 65536 7929855 7864320 EBD0A0A2-B9E5-4433-87C0-xxxxxxxxxxxx F2ECCD60-9303-46B1-B193-xxxxxxxxxxxx system
/dev/sda42 7929856 8060927 131072 EBD0A0A2-B9E5-4433-87C0-xxxxxxxxxxxx DD8D25F3-92F2-4B24-9558-xxxxxxxxxxxx custom
/dev/sda43 8060928 8323071 262144 EBD0A0A2-B9E5-4433-87C0-xxxxxxxxxxxx 01573816-7EBF-4860-8AB7-xxxxxxxxxxxx cache
/dev/sda44 8323072 15267834 6944763 EBD0A0A2-B9E5-4433-87C0-xxxxxxxxxxxx E15F5427-031C-4BB0-89D5-xxxxxxxxxxxx userdata
Expert command (m for help):
Because you have a lot of binds to the root you have to modify the partitions form recovery.
First pitfall, the default recovery image for Ubuntu is not adb enabled. I used TWRP 3.0.
Second pitfall, TWRP fdisk utility is not good enough, it is the busybox variant and it has not the modification commands. So just make a copy (with dd from the system image to a file so you can mount as a loop device) of the system partition onthe SD card, mount it, create with mknod /dev/sda in this mount and chroot /sbin/fdisk
Now you can play with partitions. Just be careful not to mess anything but the four target partitions. Don't forget to put the names to the partition's labels.
All of the four partitions are ext4 make the filesystems and then you should be able to mount them from TWRP menu.
If you have just grow the system partition you should be able to getaway with resizing the fs.
You could redo the partitions and then just reflash the Ubuntu system with ubuntu-device-flash.