What size chicken wire?

I want to build a chicken run and note that there appears to be three main hole size in the wire mesh.  13mm 25mm 50mm.
I thought it a good idea to put wire mesh under the floor of the chicken house and buried under the floor of the run in the hope that it would make the whole structure Rat proof.
Could anyone suggest what the correct hole size should be for under the chicken house floor and the floor of the run and what size hole should be used for the fence surrounding the run.  I would also welcom a suggestion as to what weight wire mesh should be used.  I intent to build the run two meters high and to have a wire mesh ceiling if that’s the correct term.

get the lowest SWG (standard wire gauge) welded wire mesh you can - the lower the number the thicker the wire but ideally use a minimum of 14 or 16 gauge. There is a company called Hills of Devon (google them  ) who sell it by the roll and have next day delivery. Their prices are very good. They don't however offer theirs by the SWG as this tends to be just steel stockholders who do this, but they do state the thickness. I would if making a run go for no larger than 1/2 inch by 1 inch holes (but this is just my personal opinion and I don't have any runs as mine free range    ).

here is a link to Hills:-

http://www.hillsofdevon.co.uk/netting.html
and as you can see their prices are very reasonable for a full roll 

Hills are very good, and very quick delivery.
Can't remember gauge, but mesh size was half inch by one inch.  Anything bigger a chook could stick their head through, and possibly get it bitten off.
Try to wear gloves when cutting and handling it, and eye protection, just in case it takes a swipe at you.  It's lethal.  Best way to get it under control is to stand on the bit that you are cutting, then it can't jump up and get you!