Is there a limit on how much JSON can hold?
JSON is similar to other data formats like XML - if you need to transmit more data, you just send more data. There's no inherent size limitation to the JSON request. Any limitation would be set by the server parsing the request. (For instance, ASP.NET has the "MaxJsonLength" property of the serializer.)
There is no fixed limit on how large a JSON data block is or any of the fields.
There are limits to how much JSON the JavaScript implementation of various browsers can handle (e.g. around 40MB in my experience). See this question for example.