You can use...

reset=combat (This resets when combat ends).
reset=target (This resets when you change targets).
reset=# (This resets when you haven't pressed the macro for that many seconds).
reset=alt (This resets when you press the macro while alt (or Control or Shift, if you change the mod used) is held).

Your macro is still subject to a maximum of 255 characters.
Unless you're using some macro addon for longer macro's.