Many shared hosts put very low restrictions on the size of the files that can be uploaded through PHP. But, many hosts also allow you to create your own php.ini file in the root directory of your site. This file can override some of the servers default PHP settings. If not already done simply create a php.ini file and place in the public_html directory of your site. If the server is configured correctly, after inserting this snippet the servers default max upload will be overridden. For this example it was changed to 20 megabytes.
; Maximum file size of post data that PHP will accept. post_max_size = 20M ; Maximum allowed file size for uploaded files. upload_max_filesize = 20M
Or you may need to preface with php_value:
php_value upload_max_filesize 100M php_value post_max_size 100M