I would plant a Dwarf one. Compared to a 60-70 feet Grandfloria Magnolia. I'm guessing dwarf's would be about 1/3 of the normal size magnolias tend to get. But of course if you try to prune it and maintain it at a certain height new growth will come and make it look so ugly. And dwarfs are "if-fy" for me. Not to mention that trees don't stop growing at their expected height, they grow more usually less than 1/10 of their yearly growth rate. But choose what you want. ;)

I set out two fairly large Little Gems in 2004. They now have a 12 foot wide spread and are 15 & 17 feet tall. The 15 foot one had its top broken off in a snow and ice storm, but have recovered nicely.
I planted both in an area that a landscaper said there was no more room for anymore trees. I wanted an area screened and the large shade trees were not getting the job done.

if i had a nickle ...

MOST size estimates are at ten years ... there is ALWAYS the potential.. that at 20 years.. they can be twice as big ... nothing really stops growing at some magical height ...

the key is annual growth rate ..

if momma is a 200 foot tall tree.. never forget.. a dwarf of her might be ONLY 100 feet ... in a given time frame ..

and the difference .. in that time frame might be.. that momma grows one foot per year.. but dwarf will grow only 6 inches per year .... back to the concept of annual growth rate ...

if you were to walk up to the plant in the picture ... you ought to be able to look at the branch growth.. and see this years somewhat green bark growth .. last year semi-woody branch bark .. and the third year backs woody bark ...

and in 'seeing' that... subject to it being potted [various cultural stresses] ... that is your annual growth rate ... it will NEVER be shorter than that [excluding hyper-fertilization in the nursery ].. and perhaps.. once established in mother earth.. it might be longer ... w/o the stresses of being potted.. etc ...

now... given your heads propensity to grow hair.. in a given space.. lol .. [why have i not ever thought of this analogy before] ... when your hair [unlike me.. if you have any.. lol]... outgrows its given space/form/diameter...etc ... you go get it cut ... [or do it yourself]

its the same with 'fitting' a plant in a given space ... but we do NOT give it a hair cut..

in the plant kingdom.. we simply learn how to prune it .. for shape and form ...

and given a 15 foot space.. that would involve some selective pruning.. once a year ... its exercise.. its in the garden.. outdoors.. its therapeutic ... and its what all true.. hardcore .. gardeners do ...

finally.. as the plant grows to fill a spot ... you have at least two options.. one being to shape/guide the plant ... and the alternative.. is to open the space.. by removing the other plants.. restricting the space ... because you like this plant better ... and this is where 'the art' of gardening takes over from the science ... in essence .. you were wishing the science of plant morphology.. would allow a plant to magically stop growing a fill a given space.. and i dont care how many decades you spend searching the web .. you are NOT going to find such.. and that is the root of your frustration ...

now.. all that said.. plant what makes your heart go pitter/patter .... buy smaller than the one at the pic ... guide its growth as you wish ... and very frankly.. if in 10 years.. it becomes unmanageable... GET RID OF IT ... [crimminey.. if after 10 years your car sux.. you get rid of it ... so why not a given plant ...] ...

step back.. define what would please you no end.. plant it.. and either learn how to manage it thru pruning.. or move on ... when it outgrows the space ...

make any sense??

ken

Magnolias tend to always want to grow larger and larger no matter what the original descriptions. For instance, my M. 'Little Gem' is now 30'x 12' with M. 'Kay Parris' a newer selection of smaller M. grandifolia now available. But here are a few interesting smaller magnolias: -1) M. soulangiana 'Kiki's Broom growing 3' x 5' -2) M. soulangiana 'Milliken' 5' x 5' -3) M. liliiflora 'Mini Mouse' is an extremely slow growing 'miniature' species selection -4) though M. 'Genie' may grow over 10' tall it is extremely fastigiate growing only 4' or 5' wide -5) on that same line, M. 'Judy Zuk' may grow quite tall, but it also grows nicely in tight spaces with limited width (8'? wide). Check the Magnolia Society International web site for other cultivars. This small group of university researchers, nurserymen, gardeners and directors of private & public gardens meet annually throughout the world (2018 Raleigh, NC, 2019 Chicago, 2020 Korea, 2021 USA?, 2022 Germany, etc.).

Saw a 'Little Gem' in the Los Angeles area that was about 42 ft. tall.