What do you mean by “best setup”?

Is it for tv sound with the Arc or for music? 

I would personally go with the larger Sonos Fives for that size of room, not much point in using the smaller Era’s with a high pitched roof anyway. 

I’d be tempted to go with an Amp and the Sonos / Sonance outdoor speakers mounted high enough to be out of the way and angled down into the listening area. You can put three pairs on a single Amp and that should do a decent job in that space.

https://www.sonos.com/en-us/shop/outdoor-speaker-pair

If you want it really loud a Port and third-party amplification and speakers is a possibility.

I have a TV in the gym for the odd music channel /sports but 90% of the time it’s streamed music, I mainly use Spotify 

If your choices, I’d go with the Fives, set up as Room1 and Room2 and then grouped.

@Stanley_4‘s suggestion also has merit. 

Thanks for the suggestion, i’m leaning towards the 5s, not keen on the Sonance speaker, i dont think they are actually made by Sonos  

The Sonance speakers are made by Sonance with a special Sonos design so that they are more compatible with the Sonos Amps.