#Sed, 1 byte

d

As sed requires an input stream, I'll propose a convention that the program itself should be supplied as input.

$ sed -e 'd' <<<'d' | wc -c
0
$ sed -e '' <<<'d' | wc -c
2

An alternative program is x, but that only changes from 1 to 2 bytes of output when deleted.

Sed, 1 byte

d

As sed requires an input stream, I'll propose a convention that the program itself should be supplied as input.

$ sed -e 'd' <<<'d' | wc -c
0
$ sed -e '' <<<'d' | wc -c
2

An alternative program is x, but that only changes from 1 to 2 bytes of output when deleted.