Output ls -l size field with digits grouped by thousands?

Block size - GNU Coreutils says

A block size specification preceded by ' causes output sizes to be displayed with thousands separators. (Note well that just specifying a block size is not enough).

So depending on what you want, you could try

BLOCK_SIZE="'1" ls -l
BLOCK_SIZE="'1kB" ls -l

or

ls -l --block-size="'1"
ls -l --block-size="'1kB"

you can make it permanent using

export BLOCK_SIZE="'1"
export BLOCK_SIZE="'1kB"

or

alias ls="ls --block-size=\"'1\""
alias ls="ls --block-size=\"'1kB\""