I think you are limited by allocated physical memory, which is probably quite a bit.
Another alternative is to use the curl library to send data as a POST instead of GET.
Yet another method is to use a database connection as a pass-through. That, or if you can write to a file, write the data to a specific filename that the other script will look for.
The database or external file method comes in very handy when you are moving from http to https. Many ISPs host secure transactions on another server, then use mod_rewrite rules or some other black magic to make it look like it's all on the same server. Of course, the POST and/or SESSION information get hosed in the process. Using GET, a database, or an internal file as a pass-through gets you around the multiple server issue, each with their own pluses and minuses.