Hardware support rakwireless RAK636


I try to build my own openWRT Image for a RAK7289V2.
Looks like they have only 3 Firmware Images: RAK 633, 634 and 636
So from 64MB RAM to 128MB and from 8MB Flash to 32MB in 633 to 634. The CPU is the same. For the RAK636 i didn't found a datasheet, but looks like the same flash size just a bigger firmware image and a newer Kernel.

For building my own Image I think I have to modify the mt76x8.mk makefile. How do I have to set the IMAGE_SIZE Variable? Or how do I find out what I have to set?
The original Firmware has uImage and squashfs, so I think I need the full image size?