There are a number of ways to do this and none of them are really related to osTicket. You should consult with your hosting company to find out if they support custom php.ini files or .htaccess .
The basics are you would edit the php.ini and add or change:
upload_max_filesize = 10M
post_max_size = 10M
memory_limit = 12M
Generally speaking post_max_size is the limit of everything submitted with post, so many people chose this to be a multiple of upload_max_filesize to allow multiple files to be uploaded, but this is not mandatory.
If your host does not allow custom php.ini files you may be able to achieve the same results by creating or editing your .htaccess file located in the root osTicket directory.
php_value upload_max_filesize 10M
php_value post_max_size 10M
php_value memory_limit = 12M
