ViburnumValley said:NMoasis damns me with faint praise...
I sense a contradiction in the request and requirements from TreeSong. The ultimate finished product needs to be no taller than "x" and no wider than "y" - yet there is a demand for a fast growing plant! Those two components will constantly battle, and create permanent work for the gardener to maintain a static situation.
Rather, I would suggest investing in a plant that has more restrained habits - especially width, since you have a very sensible wish to have ground space for additional plants. The height issue should be cast aside, since you plant to prune it to a perceived set height anyway. There are many selections of narrow plants, and it is easy to do online searches or survey opinions here.
The first plants that came to mind for me were choices like Ilex crenata 'Sky Pencil' - a very narrow Japanese Holly selection. It is made for zone 7 New York. Another is Ilex crenata 'Jersey Pinnacle'. There may be more of these Japanese Holly selections of which I'm not aware.
Another group of plants not yet mentioned are the Boxwood clan - Buxus sp. Not fast growing, but there are narrow selections of this group as well, so maintaining the height you like will be the only effort required.
@sallyg: any chance your neighbor's hedge might be Euonymus japonicus? That might be a lot easier to maintain smaller - and I'm not besmirching your ID skills. Just that heavily sheared plants can mask identity when quite similar to cousins.