this is a php.ini settings issue. the following page details the php.ini settings that will effect the maximum file size:
wap_upload.asp?page=php2
bear in mind that there may be client limitations as well.
If the client connects to the internet using a slow connection, it is going to take quite a bit of time for a large file to upload to the server. this will need to be accounted for when setting the max execution time.