/**
 * Enforce minimum image upload size.
 */
add_filter('wp_handle_upload_prefilter','handle_upload_prefilter');

function handle_upload_prefilter($file) {
	$img=getimagesize($file['tmp_name']);
	$minimum = array('width' => '640', 'height' => '480');
	$width= $img[0];
	$height =$img[1];

	if ($width < $minimum['width'] ){
		return array("error"=>"Image dimensions are too small. Minimum width is {$minimum['width']}px. Uploaded image width is $width px.");
	} elseif ($height <  $minimum['height']){
		return array("error"=>"Image dimensions are too small. Minimum height is {$minimum['height']}px. Uploaded image width is $width px.");
	} else {
		return $file; 
	}
}

Add this snippet to your Wordpress theme's functions.php file to reject any image uploads that are below the specified dimensions.

Enforce minimum image dimensions in Wordpress

wordpress, images

PHP