How to stop a oversize file upload while upload is in process?
max_execution_time and max_input_time These settings define the maximum life time of the script and the time that the script should spend in accepting input. If several mega bytes of data are being transfered max_input_time should be reasonably high. You can override the setting in the ini file for max_input_time by calling the set_time_limit() function in your scripts.
This is taken from http://www.radinks.com/upload/config.php
If you set lower values for these ini entries the script should timeout before the upload has completed
It's not possible to stop this from the server side - the file will still be uploaded before PHP decides whether to reject it. You'd need to add that detection on the client side. While you can't do this with a regular HTML form, flash uploaders can find out the filesize before the upload starts.
On the server side, you can't do anything useful with PHP - the files have always already been fully uploaded before PHP has a chance to say 'no'. To block upload (that is, disconnect the client part way through the POST), you'll need some web server magic lower down the stack. Perlbal is capable of doing this. Writing your own Apache module is a (brute force) solution. Using any kind of lightweight proxy in front of your main web server is the solution if you're worried about big uploads tying up web server resources.
It really depends why you want to limit large uploads. If it's to save users time, just add messaging that specifies max file size. If it's server resources, use a lightweight proxy in front of the server. If it's something else, well, what is it?