Big real model and the size of the multi target

Hi, for a demo, I'm using iPad2, Vuforia and Unity3D to show a real size model of ultrasound machine. These are approx 1.5m x 1m x 1m so the viewing distance is relatively big. The models should be changed at runtime, and the user should be able to walk around, zoom in by coming closer etc. Also, the height of the model should be appropriate, so the user needs to duck to see e.g. the wheels in detail. For this, I've been using multitarget (cuboid), physically a box approx 1.2m x 0.4 x 0.3 (I couldn't find a bigger box). The tracking images are bit difficult to find. I am able to print A0, so I just used some complex images with 4-5 star rating in TMS. However, the tracking performance is not very good. Once the tracking is lost, the recovery is quite lengthy. Also, the tracking seems to perform better from distance (~2-2.5m) than close up. I've been trying to experiment with different images, also modifying the XML by adding more image target in my multi target (e.g creating cuboid with more "layers"), but it did not seem to help. Any help will be appreciated! Thanks matej