In most home computer BASIC interpreters, the source code (including line numbers) never exists as text in memory or on tape/disk, only as binary tokens. The only place where the original input text exists is a small line buffer which gets translated into binary token form when pressing Enter/Return.
The LIST command translates the token form back into human readable text.
And if you use 'AUTO' you also never type any line numbers... so are they actually part of the source code? Is there even 'source code' when the original source is never stored as text anywhere? Is the 'source code size' then the size of the original text data (that actually never exists a whole), or the size of binary token representation in memory and on tape/disk? :)
5 REM MY COOL PROGRAM
10 PRINT “HELLO”
20 GOTO 10
10 PRINT “DIE BART DIE”