I got this information from one of our very knowledgeable horticulturists here:
Look at the Cityline series of Hydrangeas - each one is named after a city. They are a dwarf hydrangea series, growing 24 - 36" at maturity. Philadelphia is hardiness zone 7. There are numerous online sources or you might try calling Mostardi Nursery in Newtown Square or Primex Garden Center in Glenside.
Cityline series information here
and if you Google Cityline Hydrangeas, you will find more information including videos.
Sounds like you are working in a small space area. We do have a subject guide on this topic
Urban Gardener : Gardening in Small Spaces
This subject guide is useful in finding information about gardening in a small space, both indoors and outdoors