Some shrubs grow through their life...others get only so big and no more.
What plant are you talking about?
AND....to preserve shape and flowering, shrubs need to be pruned.

It will depend on the type of shrub. Like most plants, they have a genetic potential for size. Once they achieve that, growth continues but at an almost infinitesimal rate.

And yes, you DO find mature shrubs that are 20' across or 20 or more feet tall. There are rhododendrons in my garden that were planted in the 60's that are easily that size. And English and Portuguese laurels, photinia and Pacific wax myrtles will easily reach that size in time......and not all that long a time :-)

btw, pruning is never a requirement and many flowering shrubs are seldom, if ever, pruned as they simply do not require it.

I‘m growing callicarpa dichotoma, buxus microphylla, hydrangea serrata, euonymus alatus, thymus quincecostatus, and going to grow azalea kaempferi, leucothoe keiskei in the future.. and Especially with the thyme, it grew 3.5 ft across in just 2 years from a tiny 3.5 in pot. By observing that kind of rapid growth rate, I can’t imagine the thyme not being 20 ft across one day..

Well I do hope all my shrubs have the genetic potential to slow or even stop their growth because I am horrified at seeing some of these growth rates of the shrubs I am growing and it hasn’t even been a few years!

I remember the first time that I read Daphne DuMaurier's 'Rebecca' and the protagonist spoke of the rhododendron blossoms far above her head. I was impressed.

What everybody else said... Where I grew up in NJ--and going to the Main Line a lot--there were many old places that had been in families for generations, and some of the shrubs like rhododendrons were enormous. We, ourselves had some very old azaleas and a gigantic forsythia that had been kept as a tree. The single stalk forsythia was about the size of a Japanese Maple... The biggest azalea that we had did require pruning of dead wood or it would accumulate a ton of it and wind up looking awful.

The NY Botanical garden has spireas snd lilacs the size of houses...

Ha, Addison, I grew up on the Main Line. We had just such a rhododendron at our house that was over 70 years old at least. I so wished I could have taken it with me when we cleaned out the house. The new owners got rid of it! Broke my heart-it was beautiful and bloomed like mad.

I'm so sorry! It hurts to lose them! ...Our azalea and the forsythia were destroyed in Sandy when an ancient pine lost a limb the size of a tree on top of them, and I am still not quite over it.