Salvete sodales!
Here my question on an example. You draw a circular ward or a spell with duration circle with a stick on the ground. Does the spell end when you put forth your arm out of it?
Thanks! 
Assuming a standard Circle target and Ring duration:
The spell ends when either the target steps outside of circle or the ring is broken.
I tend to apply that rule literally, so waving your arm across the boundary won't end the spell. What I don't allow is for combat across the boundary without breaking it.
Exactly. The problem is that the term "broken" is not defined in canon, and as a result there are almost as many opinions and interpretations of what constitutes breaking as there are SG's/Players.
Some rule that anything crossing it (a hair, a thread, a twig, an insect, the hem of a robe) breaks it, others that it has to be physically disconnected and/or completely erased (which leads to some rather "unbreakable" prepared rings made of magical metals/etc.). Some say the circle/ring forms an infinitely high cylinder, some that it's a sphere, some say it's only the circumference of the sphere, the actual circle/ring itself that matters. Some have rulings about how large it must be, that "tiny" circles cannot be used for some purposes. Some rule that it cannot be mobile, that it must be fixed in place, while others that it can be mobile (perhaps with some restrictions on "size" - a ship yes, a push-cart no - it varies widely). Some that it can be vertical (like on a wall), others that it only be "flat", on a floor/etc. Some rule that it must be rigid, other that a ring on fabric/cloth/leather(/skin?) is perfectly acceptable. (Have I forgotten any other variables? Probably...)
Some of these answers can be drawn from "classic" sources, the genre itself (i.e. your interpretation/understanding of it), and some can be manipulated to address how you want your Saga to run. The differences lie in how useful/restrictive/creative/abusive wards can be. As a SG, you should take every variable (that you care about) to a logical extreme, and consider the implications for your Saga.
[i](Edit - on afterthought, that gives you only the answer that you are not alone in wondering what is "right", and that that can only be answered by you for your saga. However, I'll toss out what is right for me for my saga, and you can adopt, adapt, or just trash outright.
First, I like my wards to be "spheres", because it's consistent with the existing wards vs. Faeries in the book and the other alternatives can create headaches. I rule that anything falling across the circle that is large enough to bridge both sides of the "line" breaks the circle - for my money, circles are fragile and unreliable things, and creating a "permanent" one is for convenience, not invulnerability. They can be mobile if large enough to hold a man. If a vertical circle/ring is preferred, or even one on a ceiling, that's fine - draw it, and it's yours to use as you can, but it must be as flat as possible to get the drawn line as close to a circle as possible (tho' there are ways around this). They can be on fabric or skin, but that makes "breaking" (which would then include distortion of the circle/ring) considerably easier.
They are not a substitute for Duration:Permanent magic items, but can be a cheap alternative if done right and a mage is willing to accept inevitable recasting.)[/i]
The advantage - from a game balance point of view - of defining the ward as a half-sphere with a height equal to the radius is that small circles will be too low to stand up in! So casters will have to draw a large circle to take advantage of the height. And this is (potentially) more difficult to do than small ones. E.g. a ward just for yourself would need a 4 pace diameter for that 2 pace height needed, yo8u can't just draw that tiny circle just big enough to cover the width of your shoulders.