Is that the size of your summertime entrance or has the entrance been reduced for the winter? That times two, one on either side of the hive?
I have a problem with mice and I would like to make the fix permanent, just build it into the hive. That would work. I'm pretty sure one of our mice wont fit through that entrance.
Hey bud that is a standard langstroth entrance around 10mm high and 70mm odd wide. They like small entrances generally if its too big they close up these they tend never to modify which would tell me they are happy with how they are. Mice will not get in here for certain. Cheer$;)