image sizes for all blogs in multisite

If you look at the source of the get_option() function (http://core.trac.wordpress.org/browser/trunk/wp-includes/functions.php#L274), you’ll see that you can use the pre_option_* filter to override/customise the value for an option.

For example, the following code will cause get_option('medium_size_w') to always return 222:

add_filter( 'pre_option_medium_size_w', 'my_override_medium_size_w' );
function my_override_medium_size_w($size) {
   return 222;
}

You can use similar code to override the other image sizes too.

If you put this in your theme’s functions.php file, you’ll be able to force specific image sizes for all sites that use that theme.