Restrict the maximum file size as well as the maximum allowed resolution for images uploaded to WordPress.

wpse67107_restrict_upload.php

<?php
/**
 * Plugin Name: (#67107) »kaiser« Restrict file upload via image dimensions
 */

function wpse67107_restrict_upload( $file )
{
	$file_data = getimagesize( $file );
	// Abort when we can't get any info:
	if ( ! $file_data )
		return $file;
	// Abort when we ain't got an image:
	$file_type = array_shift( explode( '/', $file['type'] ) );
	if ( 'image' != $file_type )
		return $file;

	list( $width, $height, $type, $hwstring, $mime, $rgb_r_cmyk, $bit ) = $file_data;

	// Add conditions when to abort
	if ( 3200728 < $width * $height )
	{
		// I added 100k as sometimes, there are more rows/columns
		// than visible pixels, depending on the format
		$file['error'] = 'This image is too large. Resize it prior to uploading, ideally below 3.2MP or 2048x1536 px.';
	}

	return $file;
}
add_filter( 'wp_handle_upload_prefilter', 'wpse67107_restrict_upload' );