Wax myrtles will get pretty large for 8ft width. Since I'm in zone 7, you'll have to be the judge on what will work in your zone. Oleander comes to mind as a prime candidate. Nerium oleander 'Sealy Pink' is 8'h x 6'w. Pittsosporum tobira 'Variegata' is 8x8. There is a ligustrum sinese "Swift Creek" that is a variegated with a green/grey and yellow color. It's a compact grower 5x5. I have used these for spots of color. I wouldn't plant more than 3 together, then use some dark green to tone it down. There are some crepe myrtles that are medium growers. Lagerstromia Pink Velour Whit II is 12'h x 8'w when mature. Raspberry Sundae is 15x8. Abelia x grandiflora is 8x8. There may be an illicium (anise tree) that works in your area). I don't know if the sunny spot would be too much for michela figo (banana shrub), 10x8. If you could shrubs smaller in height (no screening) Indian Hawthorne -- rhaphiolepis umbellata 'Snow White' and 'Pinkie' are 3x4. That's about all I can think of for now

I have had great success with Gaillardia's in full sun, but they do need regular water. My fav variety is Fanfare: Also, Tecoma Stans (Yellow Bells), Crape Myrtyl, Pineapple Guava Bush and Butterfly Bush. Vines: Purple Leafed Honeysuckle and Caralonia Jesamine (both of which smell great). Vitex (Chaste Tree), not sure of the variety, gets small purple flowers but has beautiful foliage with a purple underside. These are all have been doing well in my full, full sun, west facing yard! While they don't strive, my Day Lillies survive. These are all have been doing well in my full, full sun, west facing yard!

All should be okay for ~6 hours of sun in zone 5. But I'd lean toward the bottom two as being the least sun tolerant, if we're ranking them.

1818 Federal, size is caused by a combination of factors: how happy the plant is in its setting, how old the plant is, and genetics. I don't think that full sun in VA would be a good idea. It sounds like its current eastern exposure is probably good. I know that all along the east coast weather has played a part for the last few years in setting back big-leaf/macrophylla hydrangeas, with late frosts damaging opening buds or particularly severe winter temperatures killing back branches to the ground. I don't know if you noticed any type of cold damage, but this is one possibility. Did you get bloom? If so, was it in spring or later in the season? If yours is a relatively young plant, you won't find it putting on a lot of above ground growth in the first year or two as it is growing roots to support future above ground growth. Do you know the variety you have? Was it grown from a cutting of your grandmother's hydrangea? Current fashion in plants has the breeders trying to create smaller plants as many homes have relatively small yards that folks want to use for a multitude of purposes, and so often they are looking for smaller plants. So there are several H. macrophylla that are bred to be in the 2'-3' range, where your grandmother's may well have been something like Nikko Blue that can get to 6' tall and wide. If during the growing season your hydrangea is blooming and the shrub's leaves look happy (not a yellowish color or droopy) I wouldn't move it.