You can restrict the file size via the [`wp_handle_upload_prefilter`](http://codex.wordpress.org/Plugin_API/Filter_Reference/wp_handle_upload_prefilter), where

> The single parameter, `$file`, represent a single element of the `$_FILES` array.

The code below is just exemplary and untested, but you'll get the point.

    add_filter(
        'wp_handle_upload_prefilter',
        'wpse163236_restrict_upload_file_size'
    );
    function wpse163236_restrict_upload_file_size( $file ){
        $file_size  = $file[ 'size' ];
        $size_limit = '1024';
        if ( $size_limit < $file_size ) {
            $file[ 'error' ] = 'File Size Over: ' . $file_size_limit;
        }
        return $file;
    }