The wisteria standard would be good because you would be pruning it annually anyway, thus no problem with topping a natural tree. Plus, it is a fast-growing plant, so you could get the screening going comparatively rapidly, whereas a naturally small tree might take a long time to reach the desired leavel.

Nothing grows to a predetermined height and then halts, without dying or dying back (even the tallest known redwood has a dead central leader, with rather vigorous sideshoots growing around it, which will presumably die back in turn at some point, when they get too far from the ground to be kept adequately supplied with water).

I immediately thought of Rhus typhina -- fits your shape and size expectations, although it can get taller -- does sucker some but they're easily kept in control. It's more of a large shrub than small tree but has a nice small tree look. An underused native plant, in my opinion, but for whatever reason it's not that popular. It grows fast so would fill in your area quickly, has a spread about as wide as tall, and has a nice umbrella shape. I like the ones with multiple trunks but it can be easily trained to one. The cutleaf form has a more delicate look although I really like the structure and bold texture of the species. I like the strong winter structure, too, and it has great fall color. There's also a yellow foliage variety ('Tiger Eyes') that is supposed to not sucker much at all but I believe it also stays smaller than you might want (not sure about mature height but I believe it's shorter). Sumac isn't everyone's cup of tea, however, and if you don't want to deal with suckers at all it's not for you.

Japanese maples are great and I'm sure you could find one for your spot. The big disadvantage will be the slow rate of growth. The ones that stay small tend to grow much slower than the ones that are taller so it might be awhile before it fills in as much as you want.

I'm always keeping my eyes open for smaller trees, since our yard isn't that big, so I'll be interested to see what else others come up with.

Have fun chosing!

Cultivar name of new sumac is 'Bailtiger', TIGER EYES being a trademarked selling name used to promote it, not a name for the plant itself.

Thanks Ron & Carol Ann for your ideas. I do like the photos I've found of the Rhus typhina 'Bailtiger', and if it grows to 10' (which some sources indicate) rather than the 6' (which most indicate), then it could be a good candidate. The species sumac seems too "wild" for our yard (would be great for the ravine which is just outside our fence, though).

I had another thought.... If I was handy, and could build a structure which was roughly the height & width of the privacy screen I wanted to achieve, then got a fast-growing vine (such as a clematis) which would fill the screen each summer. I don't know about the winter, though... I think the tree or wisteria standard is a better idea in the long run.

Any other ideas?