Is that big enough? I think the raiders on the hive in question will completely overwhelm it?
I think it's useful to bear in mind that robbing is identical to foraging in it's principles - even though it can look like 'foraging on steroids' ...
Stage 1: is when a forager scout enters a hive and checks it out. She will then report back that there is a highly desirable source of food and, (say) poorly protected. If there is much nectar coming in from the fields, her information (in the form of the waggle dance) will be drowned-out by the dozens of competing dances. But - in a dearth - her's may be the only dance in town.
Stage 2: is when a handful of foragers checkout the source and duly report back. It is on the strength of that and subsequent information that the 'forager-workforce' takes off for the target 'mob-handed'.
Stage 3: is the cloud of highly excited foragers we call 'robbers' all intent on entering the hive - and by overwhelming any opposition, grab the spoils.
So, when I have competing interests - such as making life easy for a returning post-virgin queen - I fit anti-robbing screens of modest proportions. These are only designed to deal with Stages 1 and perhaps 2. Providing the foragers are dissuaded at that stage, then full-blown robbing won't occur. It's a trade-off between offering protection and ensuring that a queen can re-enter without any difficulty.
Ok - for dealing with modest robbing attacks (say, from my Black Bees - which are very rare), I find a 4" high wire cage with an open top is more than sufficient - here is a photo showing the template I use to cut them out.
In that shot - next to the corks - is the most effective anti-robbing device I've ever made (I've put a similar device upside-down next to it to show the method of attachment - a stub of 22mm copper tube which is simply pushed into the entrance hole) - and it works so well precisely because
it stops the robbing from ever starting. But - they are time-consuming to make. There's always a down-side.
But - if robbing should ever develop into a full-blown frenzy - what to do then ?
Here's an example of where I completely forgot (Duhhh ... ) to fit an A/R screen to a Nuc. Within hours this developed into a situation far more serious than usual, as these robbers were Italians coming in from outside the Apiary and were simply diving-in over the top of the mesh - even though theory says they're not supposed to.
As you can see I placed a temporary top over the mesh, allowing only a small hole to remain. After 2 days, this measure eventually quelled the riot, and brought the numbers of robbers down to single figures. When that had been achieved, my girls were no longer overwhelmed by sheer weight of numbers and could then set about dishing out their own brand of punishment.
In the photograph there are 3 fights taking place, where single robbers are being stung to death or having their wings torn off. One fight is taking place at bottom right, outside
the mesh. The second is taking place inside the mesh at 3 o'clock and the third up in the top right-hand corner.
What you can't see is that guards have also set up station inside the box entrance - so that three levels of guarding have been set-up - one outside the mesh, one in the 'forecourt', and another just inside the entrance.
I even began to feel sorry for those robbers ... well, not too much.
Shortly afterwards, the top was removed, as it's job had been done. Only the occasional robber tries to enter now, and the nuc guards can quite easily chase those away. Drama over.