Hardware support rakwireless RAK636

Hey!

I try to build my own openWRT Image for a RAK7289V2.
Looks like they have only 3 Firmware Images: RAK 633, 634 and 636
https://downloads.rakwireless.com/WIFI/RAK633/Hardware%20Specification/RAK633_Product_Specification_V1.0.pdf
https://fccid.io/2AF6B-RAK634/User-Manual/User-Manual-5502240.pdf
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?