The phrase "a colossal tree" is correct and usable in written English.
It can be used to describe a tree that is extremely large or impressive in size.
Example: "As we hiked through the forest, we were awestruck by the sight of a colossal tree that towered above us, its branches stretching wide."
Alternatives: "a massive tree" or "a gigantic tree".
We passed stands of wild orchids and a colossal breadfruit tree.
I stopped at the crest of a small rise and settled into a pleasant wooden bench perched under a colossal linden tree.
Darnell, a researcher in astrobiology at the U.K. Space Agency, attempts to create a post-apocalyptic "quick-start guide" that tests whether "the key to preserving civilization is to provide a condensed seed that will readily unpack to yield the entire expansive tree of knowledge, rather than attempting to document the colossal tree itself".
He was very successful in Russia but he felt in the West he would have greater freedom to work on his masterpiece--a colossal statuary group called "The Tree of Life".
As though to listen, the colossal trees lean out into the tungsten-haloed street.
Despite the country's conservation policies, which wildlife groups say are among the best in Africa, the logging men dig deeper into the rain forest each day, felling colossal trees, dragging them out with chains and shipping them out on logging roads that lead to where the elephants live.
There are colossal trees with buttressed trunks.
In the yard, behind Henry James's tree, is a colossal pit, next to which a bulldozer and a backhoe have been sitting since the summer.
Ironically, Kenton Kilmer stated that while his father was "widely known for his affection for trees, his affection was certainly not sentimental the most distinguished feature of Kilmer's property was a colossal woodpile outside his home".
