Any suggestions for where the mile-wide Reikwald Circle spoken of in Winds of Magic book (page 12) would be located?

I don't think there's previous canon on this location. I'm thinking south-west of the Hagercrybs, a few miles into the forest from the Bögenhafen - Ubersreik road, about 50 miles north of Grunewald Manor from Eye for an Eye - making it nicely situated for an Ubersreik-based campaign.

To be honest, whenever it fits for your adventure.

Even if it's located in specific place, you still can just move it a few miles in order to use on game session.