I have vandalised wife’s cake rack to make this mice guard. Which doubles as a entrance reducer.
The internal dimensions of each hole is 10mm (approx 3/8") square and I was wondering if this was too small for laden bees returning to the hive.
Wow ! You took her cooling rack ?!!! And your still alive. You must have more than one ! 
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Pretty inventive n cool ! Up here the cardboard wouldn’t last. Very wet n cold all winter !
The holes are not too small for bees to get through. They can easily get through anything above 6mm (1/4 inch or #4 hardware cloth), including drones and the queen.
My concern is whether 10mm would keep a small mouse out. Having worked with mice in a former life, it is amazing what they can squeeze through. I will measure the holes on my mouse guard (it works) and let you know how big they are:
Dawn
OK, so here is what I found. My mouse guard has holes 9mm in diameter - I measured them myself. 
The description says 3/8 inch, but they are actually slightly smaller than that.
If you read about it, pest control companies say a mouse can get through a hole as small as a pencil thickness (1/4 inch or 6mm) but I don’t believe that. Maybe a baby mouse could, but even then, it would be a tough squeeze, and a baby that size is not going to survive without an adult mother. More reasonable guides say 1/2 inch gap is big enough, which would be 12mm. So, might be good to straighten those bent wires at the bottom of your mesh - some of them could make a gap bigger than 12mm! 
All the best,
Dawn
It’s actually apiece of ply but I will replace it with a piece of tin plate.
Thanks Dawn,
Yes it looks untidy as well.
busso
If a mouse can get its head through it can squeeze its body through as well
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Exactly, and a mouse skull for an adult is about 10mm max diameter. You don’t want to know how I know that… 
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If you make your own entrance block you can make a long slot entrance 8mm high; (bee space)
That will keep mice out. You can leave it in all year if it is most of the length of the entrance block, maybe using a little sponge to close some of it off if you need to prevent robbing (wasps or bees)
I actually have my entrance blocks with a long slot one side and a short one on the next then all I have to do is turn the block.
I think you would be well served to have the solid wire running the length along the bottom instead of “free floating” wires. It looks like you have enough spare that you could clip them and do it that way.
And they are easy to make as well I bought one and made the rest to measure
Yes have done that. Tidied up and taken the kinks out. Will post a pic next day or so.
Wouldn’t the mice squeeze through the top of the wire and down to the entrance? I repaired phone lines for 30 years and would be amazed on how small a hole or gap mice could get through to get inside a phone pedestal and make a cozy nest.
I have discarded the mesh guard and installed a more traditional wooden one.
Made from some recycled Sheaok the entrance according to the depth gauge on my Vernier callipers is between 8.91mm and 8.93mm or close enough to 9 mm so less than the 10mm mesh and mouse head.
Tonight when the girls are asleep I will take it off and oil it. The Sheoak is a very durable timber still used in barrel making and left to weather outside turns to a silver grey. Can lay buried for +20 years without rot or decay.
The bees tend to find the entrance more easily now. They were getting confused landing on the mesh, finding it difficult move to the part of the mesh open to the hive. Lots of fly around till they got it right.
Now they land on the pad walk up to the hive and move along under the ledge till they can enter the hive. Much more efficient. Well not perfect I see in the picture one poor lass laden with pollen is moving to her other right.LOL. When she reached the edge she turned around and found the hive entrance.
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Beautiful wood, and fabulous workmanship. Your bees look busy and settled!
I love it. This a computer desk I made (see past the dust LOL) I did post it in another thread but can’t find it. I had some logs milled from our place many years ago and made desks and tables and bookcases till it was all gone.
With regard to the guard I am not sure the entrance is large enough. Today is the first day with any sunshine, not a lot, but best day in a few and the entrance is just packed. Went to take a photo but the battery was flat.
Dawn you must live on the West Coast or you don’t sleep.
Both? 
My body clock is very troublesome, and I travel, which challenges it even more… Oh well, at least I get to keep in touch with friends!
Hi Folks, after assembling my ‘6 Frame Classic’ hive (which fits 8 brood frames), I’m finding that the entrance gets tighter towards the back, i.e. towards the beetle mesh. In fact the space measures only 5-6mm. I have two questions - is this big enough for the bees to get through easily? Don’t they need 8-9mm? And could mice still squeeze through that space? I’m a bit doubtful on the latter. Thanks for any insights.
This website contains everything you ever wanted to know (and more) about spacing for bees…
http://www.dave-cushman.net/bee/bsp.html
Per the article, 5-6mm, is big enough for bees to fit through.