How To Customize the root file system size at Compile

Hello everyone, I am developing software on asus-ac68u, the size of the software is 20M, I found that the /usr/sbin/ in the compiled router does not have such a large space, but the flash of the router has 128M, I do not want to use usb and sd Card and the like, how can I customize the size of the /usr/sbin/ catalog or the size of the / catalog when compiling?

try Dynalink DL-WRX36 Askey RT5010W IPQ8072A OpenWrt support - #513 by slh,
do not reply in the thread, it'd be considered off topic.