You can do this using WikiTree+. For example here is the query to list all the connected profiles with Last Name at Birth Duggan (you just need to press the blue "Get profiles" button on the left to produce the list) - https://plus.wikitree.com/default.htm?report=srch1&Query=LastNameatBirth%3DDuggan+Connected&MaxProfiles=5000&Format=&PageSize=100.
You will certainly want to use the One Name Trees app that is still in beta. It groups the profiles by each connected branch showing you the branch size.
Details here ...