If it's not -s, you probably meant to know: how do I know a file size in advance when someone uploads a file?

You usually have no way of controlling that, though your web server will put a maximum length to the size of the file it's receiving. You will simply evaluate the length of the returned file (you need a module to parse it, I believe there must be something in the CGI family) and then decide what to do with the uploaded file.