They use `du(1) -ks' and the output looks familiar, but without further information the line is pretty useless:
$ make show-size 106 /usr/ports/pobj/oicb-1.2.1 While here, be consistent and more specific for `show-size'. OK? Index: bsd.port.mk.5 =================================================================== RCS file: /cvs/src/share/man/man5/bsd.port.mk.5,v retrieving revision 1.524 diff -u -p -r1.524 bsd.port.mk.5 --- bsd.port.mk.5 11 Jan 2020 12:43:12 -0000 1.524 +++ bsd.port.mk.5 12 Jan 2020 10:50:45 -0000 @@ -720,7 +720,7 @@ port's variables without guessing wrongl Displays the information that was generated by .Xr build-debug-info 1 . .It Cm show-fake-size -Print the size of ${WRKINST}. +Print the size of ${WRKINST}, in kilobytes. Used by some options of .Xr dpb 1 , suitable for @@ -756,7 +756,7 @@ Exactly like .Cm package , but affects only one single subpackage in multi-packages ports. .It Cm show-size -Prints the size of the work directory. +Prints the size of ${WRKDIR}, in kilobytes. Used by some options of .Xr dpb 1 , suitable for