REST payload max size?

As Will Hartung said, there is no limit in the standard (RFC 2616). But every implementation has its own limits. A few examples:

• Two megabytes for Tomcat (you can change it with maxPostSize)
• Two megabytes for PHP (you can change it with post_max_size)
• Two megabytes for Apache itself (you can change it with LimitRequestBody)

These implementation limits are usually just the default configuration values, and can be changed to larger ones if required.

No, it's pretty much up to the server implementation if there's any such limit.

There's a limit on the size of a URL (if you wish to put large amounts of data on the URL, like with a GET), but no defined limit for a POST or PUT.