By default the maximum file size you can upload via PHP is 2MB, but sometimes this isn't enough. Fortunately you can change the PHP settings on your account to get round this using a php.ini file and the following command:
upload_max_filesize = 8M
You can of course change the maximum upload value to anything you like, you are limited only by your visitor's internet connection speed and browser.
More information on php.ini files and instructions for creating them can be found in Changing PHP Settings.