A slice of ginseng root about the size of a nickel, or a piece around the size of a small nut is best. When ingesting as a tea one or two cups a day are sufficient. Buying the whole roots is your absolute assurance of quality. Many of the ginseng capsules on the market do not contain pure ginseng, and may include any number of fillers. Even those capsules that are pure ginseng may be made from root hairs, discards, and very young or otherwise inferior grade roots.
Extracts are also an effective means of ingesting ginseng, and are very popular in China.
The extract is made from the entire root, rather than trying to isolate any particular components. Pre-made ginseng teas that contain pure ginseng and nothing else can be a good way of ingesting ginseng. However, beware of teas containing several ingredients and claiming to include ginseng in the mix. They often turn out to contain inconsequentially minute amounts, if any. Ginseng is naturally white or off-white when harvested.