Starting point
Hostas are sized according to the height of the plant mound (not including the height of the flowers). Some growers also give a mound width size, but the spread of the plant mound can be largely controlled by the gardener. Therefore, if the plant becomes too big when mature, it can simply be split. A general rule of thumb is that a hosta will grow approximately 2-3 times its height in width.

That seem simple enought, so what is the problem?
Whilst most hosta experts agree on the actual size of the categories, there is still a fundamental disagreement over what the categories should be called. In particular, which term should be used for the smallest of the hostas - dwarf or miniature. Both terms appear to mean the same thing both sides of the Atlantic, although one is an adjective and one is a noun:

The following table highlights a selection of size classifications used alongside the American Hosta Society (AHS) definitions:

As you can see there is broad agreement over the small, medium, large and giant categories and sizes but there is less consistency over both the terms and sizes of the smallest hosta categories.

Our dilemma
We use the same logical system of sizes: small, medium, large and giant but our smallest hostas are termed 'dwarf'. However, with the introduction of ever more 'miniature' varieties we feel that rather than introduce a sub-category for the tiniest plants, there might be merit in re-naming our 'dwarf' category 'miniature'. As yet we are undecided, because we do not wish to further complicate matters. In modern parlance the term miniature suggests ever shrinking in size - think of electronics and the trend towards miniturisation in many everyday items. Therefore, our inclination would be to do the opposite to the AHS and call the smallest varieties 'miniature' and the slightly larger ones 'dwarf'!!

New additions to the collection
It is important to remain consistent but if you read the Hosta Registration lists you will see cultivar descriptions that quote leaf sizes to several decimal places! Plant registration is analogous to patent registration (if less long-winded) but such precision needs to be tempered with the realisation that hostas are plants and do not necessarily conform to tight classification. Therefore, although we do classify our hostas using their original registration or breeders specifications, we also advise our customers if we have experienced the actual growth of the hostas deviating substantially from that expected.

An article in the RHS publication The Garden, in July this year, focused on 'small' hostas using the AHS classification for their 'miniature' category. The AHS definition of miniature also looks at plant spread and suggests the maximum for the miniature classification should be 38 square cm. This helps classify varieties such as H. 'County Park' as a 'small' - although the height of the plant puts it in the miniature category, it has leaves as round as the plant is tall, so the spread of the mature plant is larger than that of a miniature. The article was a lovely introduction to some of the smallest hostas but using the AHS definition of miniature was perhaps a little misleading - there was no mention of the fact that the AHS also have a 'dwarf' category for hostas under four inches in height!