Hi,
Thanks for your suggestions that both does what i would like to achieve.
Aftwer writing the post i kept searching and found something on threejs.org that suggested this:
renderer.setSize(window.innerWidth/2, window.innerHeight/2, false);
The trick is to set the “false” argument. This looks to be a simple way to render at half (or other) resolution but still fill the dom element.
Hope this makes sense - i’m quite new to three js and JS for that matter!
I’m so pleased that there are people taking time helping out others and newbies like me.
Many thanks 
Henrik