Comments written as I saw each of the pictures...
If IMG 4618 is what the shrub looks like now, I would not do anything now.
IMG 4620/4621 shows a lot of old stems but, not necessarily dead or dried out. Hydrangea stems start green in Spring and then as Fall approaches, some of them turn dried out looking. The older stems remain looking dried out near the base and turn green near the end/top of the stem in Spring. Right now, due to the Spring pruning, you cannot tell for sure which are dead or dried out unless you very carefully scratch the skin to see if you see "green".
Since it is hard to tell which stems are dried out/dead and which are just plain old/pruned and since doing more pruning could result now in no flowers for 2019, I am going to suggest leaving the stems as they are. Then in May, prune stems that have not leafed out but prune tem all the way down.
IMG 4622/4623 makes me want to cut some of the dead looking stems all the way down but, since they may be alive, I would wait until May. Or if you want to very carefully look for green near the bottom of the stems, decide to prune based on whether you find green or not.
IMG 4624 - I usually suggest leaving the flower for winter interest, specially in places that get a lot of snow. The browned flowers break the monotony of all white after snow has fallen. Mother Nature then makes the blooms fall on their own between January and May. If you also want, you can use the spent blooms as mulch, in a way, recycling the aluminum that turns the blooms blue. The size of the blooms does not appear abnormal. Inflorescences range from 3" to 10". The size depends on the cultivar, age of the plant, vigor, available minerals in the soil and your pruning practices (the flower buds develop at the ends of the stem but if you prune the stem, the plant may develop one or two branches there; if it develops two branches, each one of those ends can develop flowers. Prune a lot and the plant may produce more but smaller blooms).
Should you wish to "start from scratch" and prune all the stems all the way down, you can do that but, I suggest that you prune when the plant is dormant as pruning can trigger growth.
Rejuvenation pruning is a technique in which stems of large/old hydrangeas are cut in thirds. One third of the stems (the longest) are cut in Year 1; the next longest third of stems are cut in Year 2 and any others that are left can be cut in Year 3.
Be advised against pruning on or after August: in some locations, new growth that occurs as a result of pruning in August will not be able to harden before cold weather arrives and the new growth may die. Also, flower buds can develop quite early. Down here, my old wood hydrangeas produce Spring 2019 flower buds by mid July. I suspect that August may be when hydrangeas in your location develop Spring 2019 flower buds.
Last edited by luis_pr
on Sat Aug 18, 2018 5:32 pm, edited 1 time in total.