How can I use all of the spaces of my router?

I have a PSG1218. I upgrade its flash from 8MB to 32MB. But the default firmware can just use spaces as small as before. I've tried to add "IMAGE_SIZE := $(ralink_default_fw_size_32M)" in the file ~/lede-imagebuilder-17.01.4-ramips-mt7620.Linux-x86_64/target/linux/ramips/image/ but it didn't work. How can I do?