With this tool i was able to extract the image and i've found there is an RK_ParamEd that allows to change the partitions size in the parameter file.
Now, when i try to re-build the firmware, it asks me to select the platform, which one should it pick?
EDIT: the author kindly answered me by email: the correct platform can be seen via the Rockchip Batch Tool in the "Support Chip" field.