Re: contact and via size
For technologies 0.35um and smaller, the contact and Via sizes are usually exact. This is because of the process. The process itself may vary the contact size but only by about +/-0.05um. This is due to the way global planarisation (CMP) is used for inter metal dielectrics and the formation of contact and via tungsten plugs.
If you vary the size outside the design rules, it would have severe implications for the tungsten plug process - e.g. it might not work at all. Some foudries allow you to change the dimemsion in one direction only, although I would not advise that as this can cause yield issues (apart from edge die seal).