[eluser]Colin Williams[/eluser]
You could silently (asynchronously) post the upload form to an iframe, which figures out the size on the server and reports back via the iframe. Although, it sounds like you want to avoid the initial upload altogether if the file is too big.
A viable solution might be to use ini_set() and lower the max_upload_size setting for this particular process (in the particular Controller method that handles the upload).