Currently, it seems there is a 2GB cap on uploads, but there are several things that can interfere with that:
In your PHP.INI file:
upload_max_filesize
post_max_size
These should be the same, the files are technically "POSTed" and that directive will regulate the file size.
There are also methods of expressing the file size limitation on forms (which I presume you're not doing, as it is a bad idea).
Further more, the PHP code itself may be hard coded to prohibit file uploads of a certain size, or the allotted partition may not be large enough to handle the files.
Overall, any large files are best left to FTP uploads, even though PHP 5.6.0a1
adds support for filesizes larger than 2GB.