Here's an area where we are especially grateful to our mentors for all their groundwork!
Check reference books or search the Internet for objective sources. (Search GardenAtoZ.com -- if we've written about it we've reported its size and/or included photos!)
If there is time, visit an arboretum or other public garden. Standing next to a ten foot tree is quite a different experience than reading that number. Ask at the garden's office or information desk whether the plant you seek is in their collection, where on the grounds it's planted and how long it's been growing there.
In all cases, do your research using the plant's scientific name, including the variety name. (Variety is also called the cultivar name. Plant nomenclature is fully described in What's Up 7.) Omit part of a scientific name and you can end up 'way off base. For instance, Hinoki falsecypress (Chamaecyparis obtusa) grows to 100' in the wild and well over 40' in cultivation, but its varieties such as 'Nana Gracilis' may grow only inches per year and remain garden-sized for a human lifetime.
Check a book's introductory section for an explanation if its author uses shorthand terms "slow, medium and fast" to describe growth rate, rather than specifics such as "ten feet over 15 years" or "8-12 inches per year." Often, but not always:
slow = under 12 inches per year
medium/moderate = up to 24 inches per year/
fast = over 24" per year
Below: Dirr tells us that the dwarf lilac Syringa meyeri can be expected to reach 4 to 8' in height at a "slow" growth rate (less than 12" per year). Harrison Flint's Landscape Plants for Eastern North America provides a diagram predicting that dwarf lilac will be about 3' tall after 5 years, and probably top out at under 6' in 12 years.
The plant charts in Rich's Foxwillow Pines' catalogue tells us Chamaecyparis pisifera 'Golden Mop' will reach 3' in 10 years so we expect about 4" growth per year.
Some catalogues are good reference books. All too often, however, commercial growers under-report size to insure sales even to buyers with limited space, or because they operate under an assumption we don't accept, which is, "No one grows anything long enough to see its full size."
Check a catalogue's listings against real plants you know or trusted references, to judge reliability. We've scrutinized and embraced Rich's Foxwillow Pines Nursery' dwarf conifer charts, as an example.
Many Internet sites offer reliable growth information. For instance, the American Conifer Society reports growth rate for many standard- and dwarf conifers.
Find these resources by searching for the full scientific name plus "Growth rate." Check .edu (university Extension) sites and .org plant society sites before nursery catalogues at .com