The genus Phlox contains annuals, perennials and sub-shrubs and these categories can all have small members.

The most popular annual type is Phlox drummondii and is used in bedding schemes and summer patio baskets and containers etc. It does have some very short growing selections.

There are several low growing perennial species too such as the lovely Phlox divaricata.

Within the sub-shrubs there are again several low growing forms that are often found in the alpine section of garden centres and include the very familiar Phlox subulata and Phlox hoodii.

Basically depends which type you mean?

Thanks' Fractal. I obviously have to research more.