Question How can I move a Gameobject to sit with screen size in AR

You will need to find the closest distance Z at which the bounding box of the game object does not intersect against the camera frustum. A simple brute force solution would be to subdivide the space and check to see if the bounding volume intersects the rays representing the frustum edges until they no longer do. A more elegant solution would be determine the rate of change in the farclip plane's area and apply it as a function of Z then compute the limit as the area approaches the area of the largest bounding box face on the game object.