There is no specific answer to this question, because RFC 2616 (HTTP 1.1) does not specify a maximum size for the data portion of the POST command and as such it is undefined behavior, left to the implementor.
when writing code that consumes HTTP, everything appears as a stream, so as far as the server and the browser are concerned, theres not really such a thing as length.
That said, the browser and the Web server stack almost always make choices on how much to limit post size. You will have to look at your server platform (IIS/Apache/NGinX, etc) to determine what it is for your environment. Runtimes like PHP and ASP.Net both implement limits as well. The primary motivation is as protection from dos attacks, which either consume bandwidth, hog CPU, or fill up your disk space. it would be trivial to just keep writing '1's to the stream and eventually fill up the servers disk were there not such a limit.