openwrt/target/linux/mediatek/image/Config.in

config MTK_BOOT_PARTSIZE
       int "Boot (SD Card/eMMC) filesystem partition size (in MiB)"
       depends on TARGET_mediatek_mt7623_DEVICE_bpi_bananapi-r2 || TARGET_DEVICE_mediatek_mt7623_DEVICE_bpi_bananapi-r2 || TARGET_mediatek_mt7623_DEVICE_unielec_u7623-emmc || TARGET_DEVICE_mediatek_mt7623_DEVICE_unielec_u7623-emmc
       default 32