The Micro scale category sort of expands on both the dio and model themes. The Battlefield subcategory should be built on at least a 48 by 48 baseplate and is your chance to depict a battle full of tanks, warplanes, artillery guns, etc all built to microscale. Do take advantage of the opportunity to include landscaping, roads, rivers, maybe trees, and/or buildings. The Model subcategory can be a model of any real life weapon of war. There is no strict rule for what scale it should be, aside from significantly smaller than minifig scale. As a rule of the thumb, if a person would be taller than about one brick in relation to your Micro model you are probably building too large.
This was recently discussed on LAML radio.