I think the stadium at Ahmedabad in India is the world's largest cricket stadium (in terms of occupancy and capacity). Who decides how big a stadium can be? How are the dimensions set for the pitches?
First, some terminology: the cricket pitch is the area on which batting and bowling happens and is defined by Law 6 as being 22 yards long by 10 feet wide. I'll assume for the rest of this answer you mean "playing area" rather than "pitch".
The size of the playing area is not defined by the Laws of Cricket; this is both a historical hangover from when cricket was just played on the village green, and an acknowledgement that it isn't practical to create one set of regulations which both manages to handle both a match being played on a dirt field in India and a test match at the MCG. That said, specific competitions can (and often do) define what is acceptable in that competition; for example, the current (2021-3) World Test Championship playing conditions specify
The aim shall be to maximize the size of the playing area at each venue. With respect to the size of the boundaries, no boundary shall be longer than 90 yards (82.29 meters), and no boundary should be shorter than 65 yards (59.43 metres) from the centre of the pitch to be used.
You can refer to the playing conditions for any other competition to determine their regulations.
Stadia are of course even less regulated than the playing area. You can build whatever stands etc you like - there is no requirement that the stands are within any distance (minimum or maximum) of the boundary. But if you make it too silly, fans may be disappointed and not turn up!