The number of bytes that are loaded at one time before calling the progress event.
First available in version 3.5 build 314.
Available only in ActionScript 3.
Property; The default is 8192. This differs from the URLLoader default of 65536. Specifying 0 resets the default value. Specifying -1 gets the file synchronously so that the bytes are available for the next line of code. If an error occurs when trying to load a file synchronously the value returned will be 0 and the ioError event will be triggered prior to the load() returning.