This is something I've been looking into but cannot find concrete info on how to do it.
When I write the openwrt image to the sdcard, this is the command I'm using
sudo dd if=r5c-2024-02-15.bin of=/dev/sdb bs=4M status=progress
From what I've read, the dd command writes the OpenWrt image directly to the SD card, but it only writes as much data as is contained in the r5c-2024-02-15.bin image file. This means only a portion of the SD card is used, specifically the size of the OpenWrt image.
But, I've yet to find enough information to know how to fix this. I'm using the sd card first but eventually want to run the OS directly off of the built in eMMC but I need the full space so I can remotely use sysupgrade as needed.