Shape and height reveals width
If a source tells you shape (also called "habit") of a plant species or variety and its height, then you can figure the width. Round plants are as tall as wide, and that ratio holds for three foot mini shrubs like dwarf boxwood as well as for 8- or 10-foot viburnums.
In catalogs and plant encyclopedias you'll also see descriptions such as "upright spreading" which means there is a trunk or narrow base beneath the spreading top.
"Upright vase" is another commonly used term for a plant's habit. It's an inverted pyramid, and as for pyramidal plants, figure the width at the top of an upright vase as about 1/2 its height.
Do not assume related plants are the same in shape (or in size).
There are pyramidal spruces, globe spruces, weeping spruces and dwarf spruces. So it goes almost across the board among related plants.
As an example, you already saw a round viburnum, V. sargentii. Here is an upright character from the same clan, the Japanese arrowwood (V. erosum) -- taller than wide.
Know what shape you're selecting for and design options increase. On a wet site, for instance, the moisture loving standard Fothergilla can be a size- and shape matched stand-in for this similarly shaped but moisture sensitive Sargent viburnum (V. sargentii).
The easiest pruning you can do is to simply enhance a plant's natural shape. If it wants to be round, let it be round. Let pyramidal plants have points, and so forth. Even if you are keeping a plant smaller than it would be on its own, it will require less cutting, less often if you accept it for its genetically driven habit.