For changing an object to a creature, true polymorph says:
Object into Creature. You can turn an object into any kind of creature as long as the creature's size is no larger than the object's size and the creature's challenge rating is 9 or lower.
What is stopping a player from transforming say, a mountain, into a goldfish? Are there any rules or restrictions on spells that I don't know about?
What is the upper limit for the size of an object that true polymorph can change?
Spells do only what they say they do. If there was a restriction on the size of what could be transformed it would say so explicitly. There is no limit stated, therefore no limit on object size exists.
Do remember that this is a 9th-level spell, and spells of at this level have enormous and game-changing power.
The real limit here is on what counts as an object. The DMG provides some vague guidance:
For the purpose of these rules, an object is a discrete, inanimate item like a window, door, sword, book, table, chair, or stone, not a building or a vehicle that is composed of many other objects.
Looking at the examples, there is no rules reason you could have not have a gargantuan chair or table or door and have that thing still be an object.
However, there are some things that clearly don't fit into the definition. Mountains aren't discrete (they are literally part of the earth) and they are composed of many other objects. A mountain is not just one big chunk of rock, it is millions and millions of them along with plants, animals, maybe humans, and many other materials. However an enormous statue made of rock - probably an object.
In the end, there is not a whole lot of specific guidance so you just have to run the idea by your DM and see what they think. Using common sense and the guidance in the books an agreeable decision can be made.
