Limit Image Upload Size in WordPress

There are situations where you want to limit content editors from uploading images beyond a certain size. You can even be more specific and allow uploads of a specific pixel size. Add the following code to your functions.php file:

/* 
 * Check image resolution (px) before crunching
 */
function savvy_validate_image_size( $file ) {
    $image = getimagesize($file['tmp_name']);
    $minimum = array(
        'width' => '460',
        'height' => '460'
    );
    $maximum = array(
        'width' => '1280',
        'height' => '1280'
    );
    $image_width = $image[0];
    $image_height = $image[1];

    $too_small = "Image dimensions are too small. Minimum size is {$minimum['width']} by {$minimum['height']} pixels. Uploaded image is $image_width by $image_height pixels.";
    $too_large = "Image dimensions are too large. Maximum size is {$maximum['width']} by {$maximum['height']} pixels. Uploaded image is $image_width by $image_height pixels.";

    if ( $image_width < $minimum['width'] || $image_height < $minimum['height'] ) {
        $file['error'] = $too_small;
        return $file;
    }
    elseif ( $image_width > $maximum['width'] || $image_height > $maximum['height'] ) {
        $file['error'] = $too_large;
        return $file;
    }
    else
        return $file;
}
add_filter('wp_handle_upload_prefilter','savvy_validate_image_size');

You can change the minimum and maximum width and height (pixels) on lines 6-12. Take a look at the broader post on image sizes in WordPress.