A "Is it best to use polished stones or rough stones for healing, and does size matter?"
Gemstones and crystals come in many forms, all forms are helpful for healing. I have found that some people prefer rough gemstones and crystals directly from the Earth for healing purposes; while others like their stones to be polished, faceted, carved, or cabbed. For many of us cost is a big factor. Gemstones and crystals come in all sizes and price ranges, making it affordable for all of us to use this healing technique. There really are no rules....
The size of healing stones is optional and open to individual choice. Small stones that are faceted or cabbed are easier to place on the body and to carry around with you. If the stone is very small you can always place it on a Quartz tabby (flat quartz crystal) which will amplify the energy. Some people like to place their 'tiny' stones in a drawstring pouch (medicine bag) and wear it as a necklace. (Please be aware that the constant rubbing together of your stones in a pouch may chip or abrade them).
Pieces too large may be awkward and heavy to hold. These large specimens can be used in any room for decoration while healing the environment of unwanted energies. I like to program my large Obsidian Sphere to 'protect' my home while I am away. I have seen very large crystal specimens used in 'group settings' to achieve a common goal. Medium size gemstones in the 1-2 inch range are comfortable for holding in the palm of your hand or to carry in your pocket. My advice is to choose whichever stone you are immediately drawn to. Just remember, whatever the size or shape; gemstones are powerful for healing.