Yes, there are theoretical upper limits on the maximum size which can be attained by any given animal group. (The below is from sauropod biomechanics researcher Heinrich Mallison at [](http://www.askabiologist.org.uk/answers/viewtopic.php?pid=15164#p15164
At a superficial level, a big animal requires a lot of sustenance for both metabolism and locomotion. In particular, metabolism gets a lot less efficient for large animals as the surface area becomes very small relative to the volume (per the square-cube law). Overheating or starvation would be problems for large animals, which would be exacerbated if the climate of the time is unfavourable (i.e. heat waves, or droughts leading to a lack of food).
Then, once you get down to an animal's skeletal structure (talking about vertebrates here), the square-cube law strikes again. Of course, as an animal's volume grows, its weight grows accordingly. However, the tensile strength of muscles and bones do not scale cubically; they scale quadratically. So it becomes harder for an animal's skeletal structure to support it as it grows.
Sauropod dinosaurs were really pushing it in terms of the size limit, and they "cheated" in many ways, so to speak. They have columnar limbs, which improves support, and they have bones lightened by air sacs, which reduces weight. Air sacs also alleviated the thermoregulatory problem. And of course their long necks are specializations for efficiently acquiring food.
The base reasons from a structural POV are that:
-Mass and therefore weight scales cubically so if something is made 2x bigger in size it becomes 8x heavier
-But the cross-sectional area of the load bearing elements (legs for example) only increases with the square of scale so 2x bigger gives you just 4x the area to support 8x the weight so, in this example, the stress (force/area) has doubled.
-All materials and structures have a limit to the amount of force/area they can sustain and at some point with increasing size will eventually fail (pancake in pure compression).
In short weight grows faster than the area over which the weight has to be spread as a creature increases in size.
Another limit is of course the presence of gravity, since weight only exists in a gravitational field. However it's worth noting that even in zero gravity there's limits on the size of a living thing, if that thing hopes to move, since as before mass increases cubically meaning forces required (for same acceleration) increase cubically but again those forces are spread over areas that is only growing with square of scale.
There's a cool book called:
"Why things don't fall down" that covers this question and lots else.
Its mostly the square-cubed law, as anything gets bigger, its volume gets bigger a lot faster than its cross-sectional area. so if you take an animals leg, and double length/height/thickness of it, its cross-section will square, but its volume will be cubed.
So basically theres a point where that animal thats getting bigger, the bone cant take the weight, and would just split apart.
Also the bigger an animal gets, the bigger the problem of overheating gets, getting oxygen to areas of the body, and just how much it has to eat.
Brontosaurus's were about as big as an animal can be, and if you notice they have short stubby legs that are really thick and short. and most of their size was a skinny neck and tail.
some animals were able to attain massive size due to oxygen availability at the time, with that many resources it was a viable evolutionary strategy to be huge as to avoid predation. If an environment were created with "unlimited" resources, large predators would increase in size to out compete each other. mountain sized animals didn't exist because that would have no evolutionary benefit within an individual's lifetime.