I've been sifting through the forum to find discussions of this subject but only found it mentioned in short hand - often refering to RAW describing some ward effects as a dome.
The question is how high a circle/ring effect is (and not only wards)? Is there a fixed limit or is it limitless?
E.G.: could a magus, or a group, use a ReCo spell to levitate within a circle - even to the height of scaling walls or cliffs?
Isn't there some reference to it being like a dome? Yes. Just found it.
"At night, from certain angles, the ring may be seen as a silver-hued dome."
ArM5, page 162.
Take that as the default position. If the players want to deviate from that they're changing the way the circle naturally works so you might want to add a magnitude or two depending on how far they're stretching things.
As you wish.
Does this mean if your a 6ft tall magi, and you draw a 3ft diameter circle for your ward as the demon attacks you, the upper half of your body isn't covered? 
Unfortunately not. It slices straight through, assuming it penetrates of course. There's a reason the Order doesn't have many giant-blooded magi.
LMAO!!!
IMS you could easily have an "elevator circle" with people being able to be able to be affected by rise of feathery body inside the circle.
Remember that even if the circle can be SEEN (and only from certain anlges) as a dome does not mean it IS a dome 
In other words, IMS it varies according to the needs of what feels appropiate. In most cases, it is more like a cilinder. There is no hard core RAW about this one AFAIK. The dome thingy is only made byu inference, but it ios not that clearly stated
Cheers,
Xavi
I agree with Xavi on this.
As per the upper limit... I'd say at most the Lunar Sphere
I must admit I had always presumed you had to be in cotact with the surface the circle covered to benefit. To prevent 'rules lawyer' types from using them to create all manner of portable weapons in tubes effectively launching weapons with infinite range.
I'd say it works the way the magus designed the spell to work.
It's ars magica, not scientia magica... the magic works the way you need it to work, so if the spell's designed to be a cylinder, it's a cylinder. If it's meant to be a dome, it's a dome.... both are circles. I have no problem with spell working the way the magus would like it work, as long as it's not abusive.
-Ben.
Quoted for truth 
Never thought about the unlimited range trick. I doubt my own gamers would like it if I suggested such thing as possible.*
Cheers,
Xavi
No, it means that if you've got a 6 foot tall magus, and he draws a 3 foot diameter circle for his ward as the demon attacks him, the upper three quarters of his body isn't covered. 
And then only if your magus is standing in the exact center.
("radius" vs "diameter" - discuss.)
Alternately, the answer is "tall enough, but no taller". Elevators are not appropriate, but working out the precise geometry of what a dome intersects may not be the best use of your RP time and energy. ymmv, natch. 8)
If I made a circle ward against mundane animals, I wouldnt expect birds to be able to fly ´over the top´ of the circle - no matter how high they flew...
If I trapped a faerie of the air inside a circle, I would expect it to remain captured... But then again you never know with faeries