I have seen many types of circuit in my time, some from big wide circuits with no hope of making the strip if the noise stops, to ones shown to me by a high time Mustang owner and A330 captain, wich was, take off, climb to 500, turn 180 deg, climb to circuit height, pull power, glide to base turn, turn 180 deg, fly final, repeat. this circuit was no more than about 100 mtrs from the strip, and flown at Bankstown airport...
The military use a similar technique, fly down the strip at circuit height, over the end of the runway, turn to downwind, 180 deg, set up for landing, and a 180 deg turn onto final.. called Initial and pitch.
the general rule of thumb i have seen in a majority of circuits is similar to the funnel technique (used by gliders)
there will be a point on your wing/strut that will correspond to a point on the ground where you will make it with the engine off in a glide approach, its usually about 2/3 to 1/5 up a lift strut, or about 2/3 out on a low wing aircraft. this point should be on the runway on your downwind leg. turn base when the runway touchdown zone is the wing chord past the trailing edge, again, the point on the strut/wing will line up with the touchdown point while flying and descending on base . turn final when needed. and yes, this will result in a curved base leg as you descend.
this works for all types of aircraft, and i have heard rumours that the 737 NG flight manual states a standard circuit is 1 mile from the runway.
basically, there is no hard and fast rule on circuit shapes and sizes.
i like to use, and teach, the funnel like technique. and factoring wind, using the funnel technique, you will hit your landing point every time..(in a glide). and like to challenge myself by flying the very tight 100 mtr circuit when i can... but as always, Airmanship should also factor in.
if you want to know more, its here...
http://www.flybetter.com.au/sitebuildercontent/sitebuilderfiles/Flybetterbooktwo2ndedition.pdf