Assuming you're on a Linux/Apache server (not Windows)
1. Create a PHP script (xxxxx.php) with some unguessable name, containing the line
<?php phpinfo(); ?>(Note: either erase this script when you're done with it, or never reveal the name in public -- it can be used by a hacker to discover internal settings that may help them to hack your site. Nor should you ever post its output in public.)
2. Run your xxxxx.php and note the value of appropriate entries, such as upload_max_filesize.
3. Create a file /home/<your account name>/public_html/php.ini (if it doesn't already exist). Add this line to it:
upload_max_filesize = 50M(or whatever size is necessary and allowed by the system). Note: do not give out your <your account name> in public -- it can be used by someone sharing your server to mess around in your files!
4. In /home/<your account name>/public_html/.htaccess, add the line
suPHP_ConfigPath /home/<your account name>/public_html
5. Run xxxxx.php again to confirm that the appropriate entries (here, just upload_max_filesize have been updated.