Of course, the smaller varieties need to be planted closer together than the larger varieties. In general, we find that planting the Large types about 30-36 inches apart will result in a "filled-in" look in a couple of years. Planting Medium varieties about 18-24 inches, Smalls about 12-18 inches and Dwarfs 6-8 inches apart will achieve the same result in most cases. Please keep in mind that some varieties reproduce faster than others and some spread faster than others too. These suggested spacings will achieve a "ground cover" look....but if you're trying to create a "specimen" garden you'll probably want to increase these spacings so that the individual plants can be appreciated without the distraction of being cheek to jowl with their neighbors.
We list plant sizes at maturity (five years or more) in our catalog. The listed plant sizes will give you a good idea of proper spacing in the average location. Remember that the amount of sun and water, as well as your geographic location effects the mature size of your hosta clump. Hostas grown in northern states will grow larger than hostas grown in the south.