edit your php.ini file - look for MAX_UPLOAD_FILE_SIZE.
IMHO, anything over a few MB httpd post is very unreliable no matter how you tweak the configuration. For large uploads (like online print shop submissions) use a java applet to perform a real FTP transfer. I am currently using jClientUploadPro. It has lots of amenities, drag and droop, multiple files, progress bar, transfer resumption, etc. Only size limitation is that most folks upload bandwidth is a small fraction of their download bandwidth. I uploaded a GB in about an hour over a typical cable connection....
just do it in directadmin.
login as admin, click file editor, click the dropdown menu and look for php.ini
Are you the administrator of this server if so you should see a link in the control panel called "File Editor" under Admin Tools. If not you'll need to have your host change this setting.
First of all, are you uploading the file through Directadmin or a website hosted on your server?
You obviously are not the server administrator, but a client of some web hosting company. You'll need to contact your provider and have them change this setting for you.