If you want a plant to reach a particular height - you're planning a hedge, for instance - then the best way to find what you are looking for is through the filters.
The filters appear in every category and section through the website, so whatever kind of plant you want, you'll see filters to help you choose.
You'll find them on the left of the screen in desktop; and at the bottom of the screen under the green Shop By filter button on mobile.
Select PLANT HEIGHT and choose the final height you want your plant to be. You'll automatically see the plants in stock that suit your needs.