personally, I think rats use floor space more than levels... I think you'd get better results in terms of odor / waste control by giving them more floorspace. The DCN / SCN mentioned above is about 2.5' x 3' as the floor space.

Level height will depend on if they are expected to climb up to the level or if you are providing a ramp for them. As long as they have a way up (ramp or climbing the sides of the cage) I'd go with about 10" - 12" from the floor. Rats will stand on their back legs and stretch upwards and I think that would give them room to do so. If you are using only hardware cloth for the levels, I would get the smallest dimension of it (1/4" or smaller) and consider covering it... I think 1/2" hardware cloth would be rough on their feet if uncovered and am unsure if the 1/4" would also have a negative effect if uncovered. I've had cages with pvc-coated wire shelves before without issue, but never used uncoated metal shelves.