Speaker set up for a 85x190 sqft riding arena

I’d set them as individual since trying to get multiple speakers to properly provide both left and right channels at the correct volume in stereo mode can be iffy in a large space.

You should have no problems as that really isn’t that many speakers. You will have each speaker in a Sonos Room (used to be called a Zone) and then all Sonos Rooms grouped together. Give each Sonos Room a good name like North-East, North-West, East, things that make it easy to tell which one you are seeing in the Controller.

Position them where they sound best to you, which isn’t really much help. I’d try the Fives centered on the short wall and the Play 3s on either side of the long wall, spaced equally away from the center to start.

That is a pretty big space for not many Sonos so adding a couple more Fives centered on the long walls and moving the Play 3s to fill in the gaps between Fives might be good.

You might consider a Sonos Amp and some of their outdoor speakers too, you could connect six to a single Amp which might be more sound for less money.

With that large an area the one thing that might surprise you is having the Group Coordinator (or the Sonos that picks up the audio and shares it back out) be one of the more distant from your WiFi devices can cause glitches using a closer one avoids. The group Coordinator is the Sonos you start putting the Group together from. You should try to have that be the Sonos with the best WiFi connection.