PG measures the model limits in verts and I believe they rated models in the game to have up to 17,000 without any problems. Of course, the less verts, and subsequently polygons, that you have, the better. We have been aiming for less than 1000 verts for smaller ships and fighters...typically between 500-800. However, the game can take a lot of verts and still play well (AT-ATs have over 3000 triangles each as I recall).