Vuforia- optimum ImageTracker size for use with large(10ftx10ftx10ft or lager) virtual models?

Can anyone who has developed handheld AR expereinces using larger virtual models share their experience? What size ImageTracker did you use?

From brief testing I've discovered that if too small, tracking reliability decreases, and conversely if too large, the user is more likely to crop the ImageTracker resulting in lost tracking.