By default, PHP configurations (and in some situations MySQL too) keep you from performing uploads of large files. Usually large means over 1 MB. This can be changed, and the way to do it isn't ))TikiWiki-specific((.
This explains how to configure PHP, assuming you know where your PHP configuration file is. If you don't, run
locate php.ini php.conf
For MySQL (if you want to store your files in the database instead of the filesystem) you need to change my.cnf's setting max_allowed_packet to something high enough.