DIY Network says:
The size of the plants you buy depends on the type of landscape you want to produce.
- Trees and shrubs generally come in 1-gal., 2-gal., 3-gal. and 5-gal. plastic containers or balled-and-burlapped.
- Opt for smaller sizes for plants that are medium- to fast-growing. If you plant them well, a one-gal. plant can grow to the size of a three-gal. plant in one growing season. If you want the look of a large plant instantly, you may want to buy the three-gallon (or larger) plant. But if you need 10 or 12 plants, then buying the one-gallon size will save you money.
- Most two- to four-inch containers are generally good buys for vegetable transplants, herbs and bedding plants, and they’re usually less likely to suffer transplant shock than those in larger containers.
- Multi-packs are good bargains too, but only if you really need six plants. Consider going in with a friend or neighbor and splitting the packs so you’re not left with too much to plant.