Files written to with the w command are created/truncated before any commands are run.

$ sed 'w created-file' < /dev/null && ls created-file && rm created-file
created-file

From the standard:

Each wfile shall be created before processing begins. Implementations shall support at least ten wfile arguments in the script; the actual number (greater than or equal to 10) that is supported by the implementation is unspecified. The use of the wfile parameter shall cause that file to be initially created, if it does not exist, or shall replace the contents of an existing file.

---

BSD sed provides the -a option to delay creating/truncating files until they are written to with the w command.

$ if sed -a 'w created-file' < /dev/null && [ ! -e created-file ]; then
>       echo The file was not created
> fi
The file was not created

The length of line-wrapping when using the l command is implementation defined.

From the standard:

Long lines shall be folded, with the point of folding indicated by writing a followed by a ; the length at which folding occurs is unspecified, but should be appropriate for the output device.

---

GNU sed provides the -l option to specify the length at which to split long lines when printing with the l command, defaulting to seventy characters.

$ yes | head -c100 | tr '\n' ' ' | sed -n l | head -n1 | wc -c
      71
$ yes | head -c100 | tr '\n' ' ' | sed -nl50 l | head -n1 | wc -c
      51

---

BSD sed splits long lines at the number provided by the environment variable COLUMNS, if COLUMNS is not provided then it splits at the terminal width, and if COULMNS is not provided and the output is not a terminal then it defaults to sixty characters.

$ yes | head -c100 | tr '\n' ' ' | sed -n l | head -n1 | wc -c
      61
$ yes | head -c100 | tr '\n' ' ' | COLUMNS=50 sed -n l | head -n1 | wc -c
      51
$ yes | head -c100 | tr '\n' ' ' | sed -n l | head -n1
y y y y y y y y y y y y y y y y y y y y y y y y y y y y y y y y y y y y y y y y\
 y y y y y y y y y y $