I tried changing under main mainboard -> rom chip size but does not seem to be helping. If I look at .config I suspect that the reason is CONFIG_BOARD_ROMSIZE_KB_256=y which doesn't change even if I change ROM size in main board.
Relevant portion of .config after changing it to 512 KB CONFIG_BOARD_ROMSIZE_KB_256=y # CONFIG_COREBOOT_ROMSIZE_KB_64 is not set # CONFIG_COREBOOT_ROMSIZE_KB_128 is not set # CONFIG_COREBOOT_ROMSIZE_KB_256 is not set CONFIG_COREBOOT_ROMSIZE_KB_512=y # CONFIG_COREBOOT_ROMSIZE_KB_1024 is not set # CONFIG_COREBOOT_ROMSIZE_KB_2048 is not set # CONFIG_COREBOOT_ROMSIZE_KB_4096 is not set # CONFIG_COREBOOT_ROMSIZE_KB_8192 is not set # CONFIG_COREBOOT_ROMSIZE_KB_12288 is not set # CONFIG_COREBOOT_ROMSIZE_KB_16384 is not set CONFIG_COREBOOT_ROMSIZE_KB=512 CONFIG_ROM_SIZE=0x80000