Odd question but I was wondering a TARDIS's chameleon field are there any limits on how big it could be? We know from canon it can get shrunk to a tiny little ornament size but can it become say the size of a planet and hide itself as say Srkwo 3 with a complete ecosystem evolving over the millenia unaware thy're on a alien timeship?
I'm not sure that there's anything in the TV series to confirm this either way. But in the novel Blood Heat, the 7th Doctor briefly materialises the TARDIS around the entire Earth, though this does put the ship under a great deal of strain. I think there may also be instances of the internal dimensions of dying TARDISes spilling out into the real world, expanding the external dimensions to enormous size (The Name of the Doctor show this to a limited extent).
But whether or not TARDISes are officially capable of becoming a planet on which a complex ecosystem might evolve, it's certainly an intriguing idea and I wouldn't let the limitations of canon stand in the way of an excellent concept!
The Radio Times short story "Birth of a Renegade" the Master's TARDIS is rather strained to take the form of a truly massive space warship [I'm guessing something on the order of a star destroyer].
In the novel "The Ancestor Cell" the Time Lord Battle TARDISes have their interior dimension mapped to their external dimension and massive spheres described as being moon sized IIRC. Again in that case blowing the exo-shell up to those sides puts strain on the TARDIS and disables some systems.
That said, if you want a planet sized TARDIS I say go for it! Just mention that its a special Type of TARDIS with an enhanced chameleon circuit and have fun.
Can it become say the size of a planet and hide itself as say Srkwo 3 with a complete ecosystem evolving over the millenia unaware thy're on a alien timeship?
I heartily agree with those who have already posted. This is such a fantastic idea, don't let something as silly as established canon get in your way! Go for it. This could provide the foundation for a wonderful adventure.
Thanks for the responses I can see several plot hooks as well from the simple "hollow earth" with a group actually finding an entrance into the TARDIS and exploring all the odd things inside their world with no idea it's an alien spaceship to a group with a TARDIS getting a distress call from a dying TARDIS that was concealed this way by a dying time lord during the time war and now the strain is getting too much after X million years and it needs help to save the primitive society that evolved on it before it collapsed. Could even be an interesting way for a group to get a TARDIS off they go to explore an anomalous planet and bamboo they're stuck as it's new companions/pets in the exploration of the universe.
I admit the hollow earth idea is particularly appealing even if it would work best in a parallel world. An expedition is founded and mounted in some modern era equivalent period say late 19th to mid 21st century By Professor Giles to explore the now debunked hollow earth idea. He proposes that the earth itself is not hollow but rather there is an extensive underground civilisation out there that may or may not have died out. Through studying the historical records of the world (or that part of it civilised people need concern themselves with) he has compiled stories about the homes of the goods, otherworldly beings, lairs if powerful magicians and mysterious anomalies such as tools far in advance of their time period such as the magic sword Excalibur. Obviously there is no such thing as magic but there is this Ruperyian society as he dubs and he believes he has finally identified where the entrance lies.
The party with appropriate NPCs sets off around the world to some seclude Isle/mountain/waste and deals with various threats of a natural variety storms/supplies/bandits before reaching their destination and entering the TARDIS (minus any NPCs who died or didn't make it inside). Once inside they can't get back out (doors lock) and they wander around the strange world (type 40s can take years to explore) as they're slowly shaped/moulded and taught enough they can repair/reactivate the atrium circuit so the TARDIS now aware all the timrlords are gone (except one renegade who annoyingly never seems to respond to its calls, sexy is jealous) can slowly withdraw and replace it's shell with an actual world.
Afterwards well why not take the explorers who helped save it on a trip through time and space?