what is the good antena length for 433Mhz modules used for low distance domestic usage?
I guess a simple wire of X% of wave length ?? or another calculation ?
  or is this more complex &amp; I need something more like an old radio
  antenna ?


Start with a quarter wave wire (circa 17 cm long) and if you get adequate reception you may experiment by shortening it a little bit but you'll find certain things if you shorten it too much: -


The signal level received rapidly falls away with moderate shortenings from one-quarter wavelength
The impedance becomes capacitive and this can be somewhat alleviated with base loading with an inductor to series tune out the capacitance.
You will start to make an antenna that is more optimized for higher frequencies and these may overload your receiver circuit if they are present in your area.