Not as big as you might think actually - especially if the run is attached to the house.
I made the mistake of building a huge and expensive run once and discovered that the cats just don't seem to need that much space. We've found that instead of 'running about' space the cats prefer to go upwards and challenge themseves (sounds obvious if you put it like that as they are cats after all but it didn't seem so at the time).
So shelves, platforms, scratchers etc are all a good investment. Decking is much better as a floor than concrete or slabs. If the run's not attached to the house a roof would be a good idea, it'l save a lot of carrying in and out if it rains.

thanks for the reply. Mine have this love for heights too. They want to climb up and up and up

So a garden pen with a dead tree in it is probably the best solution...?

Easy to spend '000s on it...maybe you could build a pen around the tree and tack the roofing wire right up to it, with a small house in the run or maybe part roof another bit of it.
I guess it depends how big your garden is and how your house is set up. I must admit it got very wearing taking the cats in and out when we had a run away from the house. I'd not even considered this when I planned the first run..

Totally agree with rajique regarding the platforms, scratchers etc!

Ours prefer to climb more than anything, We've also got some bird feeders hanging in a bush pretty close to the run.....they will sit and stalk them out them for hours.

The run attached to the house is about 4m x 3m. It provides them with more than enough space to play in, Once you've built the main run you can just add things as you go......ideas for the interior will come with time once you see how your cat utilises the run.

Heidi found a big peice of driftwood that had been washed up on the local beach, It was like a conifer style tree that had all the bark removed. I chopped it down a bit and made a double platform out of it for our stud pen.....It looks really well and he loves it, we now call it "Uproar's snatch post" for obvious reasons

If your good at DIY you should be able to knock something together that will stand the test of time for about £200-300 depending on size and providng you already have a sound base to put it on.....you can buy ready built enclosures but they're very pricey for what they are, considering it'll only you take a few days to construct in the summer and you'll get that tailored run always wanted.....a self built run is the best option IMO.

I dont mean to be negative, but in some areas, for example if you live in a conservation area you cannot just plonk a run on your house without consulting the council and getting permission.

I own my house (along with the bank!) its not a listed building but before putting up a run I had to submit details of size and positioning and purpose to my council. Luckily it allowed me to do what I wanted to do, but it cannot be extended because of the regulations on how far it is located from the road. So if you live in an old -style housing area ask the council first. Its worth it as you dont want to end up having to take it down when someone notices and you've incurred time, expense and effort in getting it up. They were fine about it in the end. I suppose its just the price one pays for living in a quaint place

Julie