In general, cube-square laws largely govern the scale of land animals, because volume and mass grow roughly as the cube of the longest dimension of the animal, while strength generally grows roughly as the square of it. There is also an issue of how you define size (weight, volume, surface area, longest dimension, etc.).
The other big factor that has historically (over the length of life on Earth) driven animal size is the percentage of oxygen in the air. More oxygen leads to bigger fauna, less oxygen leads to smaller fauna. It isn't entirely clear if this is mostly due to animal respiration or mostly due to availability of food.
Size limitations are less demanding in water (where gravity is effectively reduced) and would also be where gravity is low, because the need for a structure to support the body from collapsing would not be present.
In both land and sea examples historically, the largest animals and many of the larger animals tend to be herbivores rather than carnivores. The largest sea creatures are filter feeding whales, and the longest sea animal (a kind of filter feeding jelly fish) can have tentacles with a spread up to four times as long as a blue whale (but weighs much less than a blue whale). Among the large herbivores on land are the brontosaurus, the buffalo, the cow, the hippo, the elephant and mammoth, the rhino and wooly rhino, the giraffe and the panda.
It isn't clear if social animals like bees and ants with specialized individuals within a colony who are the only ones who can reproduce count as one animal or many animals (there is good reason to think of them as a single organism since no one individual in the colony is complete even over an entire life cycle). Ants, for example, can make up as much as 25% of terrestrial biomass in their territories (and are also often herbivores who farm their own food). One could easily imagine a variant of a large terrestrial herd herbivore that had a colonial bee or ant colony like structure (indeed, many farming operations already have some herd animals like studs who are specialized for reproduction and others who are specialized for food production). Is the fact that the intellectual action centers are decentralized in colonial animals relevant?
A similar issue to the question of colonial animals presents itself in Issac Asimov's 1989 novel "Nemesis" which involves an organism with many bodies but a unified mind connected by radio waves. A distributed computer network presents similar issues and one of the side plots in the science fiction novel "Blindsight" by Peter Watts' was that he wife was in charge of dealing with massive complex computer networks that developed consciousness. "Blindsight" also considers humans with "hive minds" that again raises the colonial animal question, and an alien organism for which the boundaries between the organism and the things created and used by the organism are vague. If the control systems of the largest skyscraper in the world became self-aware would it be an organism? If something can grow and repair itself but not actually reproduce, does it count?
Another issue related to the colonial animal one involves interdependent symbiont systems. For example, suppose you have a large filter feeder which has a moss-like growth all over it from which it sucks nutrition, or a parasite that can infect a large animal or plant or organism colony, and control it to some extent. At what point is it fair to call an entire ecology a single organism as under the Gaia hypothesis. https://en.wikipedia.org/wiki/Gaia_hypothesis
The hugest animal of all one could imagine would probably be a filter feeding in an oxygen and nutrition rich environment with little or no gravity. I could imagine, for example, a huge, lighter than the atmosphere, jellyfish-like or whale-like or airship-like shaped creature which would float around through and feed on the chemicals in the atmosphere of a large gas giant like Jupiter or Saturn, perhaps supplemented by an ability to absorb heat from its surroundings.
On the plant side there are larger organisms and the colonial issue presents itself again:
The largest single-stem tree by wood volume and mass is the giant
sequoia (Sequoiadendron giganteum), native to Sierra Nevada and
California; it grows to an average height of 70–85 m (230–280 ft) and
5–7 m (16–23 ft) in diameter. Multiple-stem trees such as banyan can
be enormous. Thimmamma Marrimanu in India spreads over 1.0 ha (2.5
acres). The largest organism in the world, according to size, is the
aspen tree whose colonies of clones can grow up to five miles long. . . .
The largest living fungus may be a honey fungus of the species
Armillaria ostoyae. A mushroom of this type in the Malheur National
Forest in the Blue Mountains of eastern Oregon, U.S. was found to be
the largest fungal colony in the world, spanning 8.9 km2 (2,200 acres)
of area. This organism is estimated to be 2,400 years old. The fungus
was written about in the April 2003 issue of the Canadian Journal of
Forest Research. While an accurate estimate has not been made, the
total weight of the colony may be as much as 605 tons. If this colony
is considered a single organism, then it is the largest known organism
in the world by area, and rivals the aspen grove "Pando" as the known
organism with the highest living biomass. It is not known, however,
whether it is a single organism with all parts of the mycelium
But, while your definition could certainly include a sessile organism, the requirement that it "be able to react to stimuli in a way inanimate objects do not" would seem to rule out many plant-like organisms. Surely a honey fungus colony or a large common rooted aspen tree wouldn't qualify under your definition. And, when it comes to reacting to stimuli, there is the question of how fast and how automatically.
Would a Venus Fly trap qualify? What about a trumpet vine that opens every day when it is light and closes when it is dark? What about something like Tolkein's Ents if it took them a full day to have a few sentence conversation? (The Portia spider which is a remarkably intelligent but very slow thinking animal is a less extreme example).