Will try to explain, how much ideal size to make good dump files.
We all agree that 512MiB is enough for user area, no matter the original eMMC size is.
The simple explanation is, with 512MiB from userarea, its already consist of important partitions to revive the phones. What important partitions is include in this 512MiB : GPT/MBR as management partitions, bootloader partitions which needed by the SoC, Android Little Kernel Partitions, Android Kernel (Boot) Partitions, unique partitions(NV/EFS/Persist).
Above partitions is enough to revive device, mostly will stuck in boot animations, and you are good to go to flash the rest using flashtool.