Not like an official database linked to the Internet such as some plant societies have. Have you checked Hortiplex Database or PlantFiles?

There IS sort of a database, however it is not online. Michael Dirr, who is typically considered the leading US authority on hydrangeas, has what is essentially a hydrangea database in his excellent text Hydrangeas for American Gardens. It contains listing of cultivars by species - macrophyllas, serratas, paniculatas, quercifolias, etc. - and gives mature sizes, coloring under various conditions, unique or special features, etc. However, it is not totally complete, as newer introductions come on the market all the time.

If you are looking for dwarf varieties, just Googling 'dwarf hydrangeas' should turn up various links and/or websites that would be helpful.

Check out the Forever & Ever Hydrangea website. They have two dwarf hydrangeas that only get 2' x 2'. They are the White Out & Together Hydrangea. The Together is maybe my favorite hydrangea!!! Also they have some other Hydrangeas that get only 3' x 3'. They will be reliable bloomers in your area!!!