"Mature size" doesn't meant "ultimate size", so, when you see a size quoted, you have to either know the time period the source is using or guess what they are referring to, if they don't tell you.

Plants grow very differently in different areas and with different conditions. A shrub that may average 10' in height in 10 years in one climate, may only average 5' in height in another climate in that time period. Frequently things that are reliably hardy in two climates will grow more quickly in the warmer climate, but this is not universally true. Soil type, precipitation, nutrient availability, and all types of other things will also make a big difference.

Plant usually keep getting larger until they die. There are "top-out" types sizes and "champion sizes" but these are not as commonly given, especially from nurseries and on nursery tags. These type of figures may also be less useful in planning your landscape.

With all that said, tell us where you will be growing these lilacs and maybe someone will be able to give you some idea of growth rate, etc (or at least what their experience has been).

I'm in NW Missouri.

Well, I see no one from your area has responded, so far. The most commonly quoted size range for 'Miss Canada' is 6' to 12' high and wide. At least a few normally reputable sources report 'Purple Persian' growing 4' to 8' tall and 5' to 10' wide, with a few more reporting sizes near the upper ends of this range. If it were me, and I was planning my landscape for the long-term, I'd probably use the upper end of these ranges as my target. If you contacted your local extension agent or someone at the Northwest Missouri State University Arboretum, they might be able to give you further advice, specific to your area.

it is my experience ... without 'renovation pruning' .. that lilac continue to grow outwards with suckers... and taller ... as far as you let them go ... if you let them ... within some bounds ... they arent going to turn into oak trees vertically ....

thru 'renovation pruning' .. you can force it into a compact size ...

plants that claim to be dwarf ... usually mean.. they have a slower ANNUAL GROWTH rate ... as compared to the more standard plant ... so if plain old grows 2 feet a year... a dwarf might grow 1 foot ...

that does NOT mean they stop at some magical size.. it just means they take longer to get to any given size ....

if you can google images ... and you look real close.. you should be able to 'see' growth rates to some extent ... and if not.. then you might try googling something like what is at the link ... note what is in the search box .... at the top at the link.. flip to the IMAGES ....

you are frustrated.. because there is no direct answer.. to your specific question ... because it is a lilac .. IMHO .... if they sucker at the ground.. like all my lilac.. they will get wider.. and wider as the decades go by ...

good luck

ken

Here is a link that might be useful: i dont have time to read them for you .... this morning

Miss Canada is not a "French" Lilac. It's a Prestonae hybrid. It flowers later than other Lilacs, late May, early June in MA. It doesn't sucker. Lilacs don't like wet feet, so if you're putting it in clay, it won't perform well. But with good draining soil, and at least 6 hours of sun per day, you should get 12 feet in 10 years.

Steve

my french.. though it does not sucker like others.. continues to put up more trunks... sorta like a crabapple ...

they will widen in that manner ...

or i am confused.. and i dont have a french ... lol

regardless.. the issue is MATURE size.. and i am suggesting there is no magical size at which it will stop growing.. w/o the owner being proactive ...

ken