First determine, at what distance (in meters) you’ll place the image in VR. Assuming around 5m, the width of the image should be around 2.5–3m (guessing based on real life conditions).
To test take a look at any photo frame hung on the wall, move around the space while moving away from the photo until a distance after which the details of photo starts getting blurred. You can consider that an optimal distance the the width of photo the width of your image. This is purely based on personal experiment. There are many studies done on this. You can refer to this Google link too https://www.youtube.com/watch?v=ES9jArHRFHQ&t=9s