First of all, thank you for all your efforts on those development jobs, codes and docs. I can't do what I want on my SBC without your effort.
Back to my question, for those who want to do something similar, a simple answer is by adding a line to config-default.conf:
BOOTSIZE="150"
Well, I would say Documentation to build btrfs rootfs image is well organized. Starting from the ~/build folder, I can always find a doc points me to another relevant one till the job done. Along the way I also learn more about Armbian. However, it is not the case for "size of boot partition". All my efforts tried via ~/build folder is in vain. It is google leads me to an obsoleted Armbian doc on github plus some guess, some luck and quite some time to test compile.
I wonder if obsoleted Armbian docs can still be organized/linked to enrich the current docs. BTW, the doc that helps me out is this one https://github.com/igorpecovnik/lib/blob/328b8d19e321afa9f26037bd45c14bfca56bc839/configuration.sh#L26
It provides me what a newbie is lacking: correct syntax (variable name, parameter unit).