Giant sequoias and redwoods manage to grow so large in part because they're extremely hard to kill, allowing them thousands of years to grow relatively unmolested. This made me wonder how big one of these trees could get under ideal conditions.
What can kill these trees? The main things that historically have killed giant sequoias are severe windstorms and soil erosion. Lightning often limits the height of sequoias by repeatedly striking their top branches of the tallest trees in the grove until no new growth is possible. Fires pose some danger, but mature sequoias have such thick fire-resistant bark that they can often ride out regular forest fires that burn smaller trees. In fact, you'll often see fallen sequoias where the core has burned out but the bark is still intact. However, recent drought and more intense fires brought on by years of fire suppression have started to kill a lot of giant sequoias.
So for ideal conditions, one of these trees would need to be in a transparent structure that shields it from inclement weather, and we'd need to put lightning rods around it. We'd need to ensure that there are regular fires to clear out the brush around the tree without damaging the tree itself. We'd need to ensure the tree gets enough water even if climate change brings extended drought.
Now that civilization has committed itself to taking care of this tree for many generations, how big can it get?
Unfortunately, scientists believe the theoretical maximum for tree height is around 400 feet. This is due to the limits of the xylem's transport system, which relies on transpiration by the leaves to create negative pressure to suck water and nutrients up from the roots. As trees approach this maximum height, the required pressure becomes greater, limiting the efficiency of the process, which limits the amount of photosynthesis that can occur. The tallest trees in the world are around 370 feet and are almost all redwoods.
However, from what I can tell, there's no physical or biological restriction on the width of a tree. Sequoias tend to prioritize growing wide over growing quite as tall as redwoods, making older sequoias look a bit like baobabs, with fat trunks and short stubby branches at the top. It's likely that under ideal conditions, sequoias could keep growing wider indefinitely, even if they are limited in their maximum height.
So with an enormous budget and a several thousand years, we might manage to grow one of the towering and intricate city-trees that seem to always show up in fantasy novels. Huzzah!