The map size for block levels is 250,000 quants diameter and the maximum texture size is only limited by the video card, normally 4096x4096 for high end cards (I haven't seen 8192x8192 yet). Every block shape available that I've seen doesn't get within these limits. The closest are the spheres that get to 26 polygons per block. The 15 edges limit is likely wrong because there's a 16-sided cylinder figure available.

Using a model-only level increases the maximum level size to a theoretical 4.19 million quants diameter with a theoretical 4.29 billion polygons per model, let alone much faster rendering speeds over 50 times faster.