download the php manual from www.php.net for a start.
In your form, you will find a hidden field called MAX_FILE_SIZE. Php has a default, but you can set your own value.
go into your form, and put the following input box if it doesnt already exist
<input type="hidden" name="MAX_FILE_SIZE" value="yourvalueinbytes">
1 byte = 1
1 KiloByte = 1024 (1 1024)
1 MegaByte = 1048576 (1024 1024)
Hope this helps. Php or should i say apache will stop any files greater than this size being uploaded! MAGIC!