I'm trying to change the ramfs size in kernel .config automatically.

I have a ramfs_size file generated with du -s
cat ramfs_size
64512

I want to replace the linux .config's ramdisk size with the above value
CONFIG_BLK_DEV_RAM_SIZE=73728

Right now I'm doing something dumb like:
size=`cat ramfs_size`
linux_size=`awk -F'=' '/DEV_RAM_SIZE/ {print $2}' .config
sed -i 's/$linux_size/$size/g' .config

It's not working from Makefiles because I think it gets confused with the $ dereference... Is there a simpler awk/sed line I can do?