you have to do this via of htaccess on your server, php.ini on your servers root, or php.ini from the server itself. are you on a dedicated box or vps? are you on a shared box?

these are all contributing factors, please respond with that information so we can further assist you.

also, please visit http://www.expertzzz.com/
sign up for an account, login to forum, run a search query on upload max_size, you will find a ton of posts over there. i am sure there are just as many on here, but see more i haven't found the search to be as user friendly on this blog as it is on the forum.

later,
DosDawg