It really doesn’t matter — don’t be fooled into spending a lot money.  Any speaker cable will do — I would never spend more than £2-£3 per metre for this.  

Size is given here, I like to go up one wire size as it can’t hurt and is cheap to do.

https://support.sonos.com/en-us/article/choosing-a-speaker-wire-gauge

Just make sure your wire is rated for in-wall/ceiling use, not just zip-cord or wire intended to be run across a room.

Not this: https://www.amazon.com/AmazonBasics-16-Gauge-Audio-Stereo-Speaker/dp/B006LW0WDQ/ref=sr_1_1_ffob_sspa

Like this: https://www.amazon.com/Speaker-GearIT-Meters-Oxygen-Theater/dp/B01N3NYB2D/ref=sxin_16_pa_sp_search_thematic_sspa