I already said that the circuit has to be powered by a watch battery (that, almost sure, it's 1.5v). And the spark, well, should be powerful enough to light some flash powder (the one you can find in regular firecrackers), so it's size is not really a problem. Plus, the spark must be somehow delayed for a couple of seconds or so, so that the audience won't realize that I just pushed the button.
Like, for example, I reach into my pocket to close the circuit, then I get my hands out and pose in a way that will make me seem that I started the effect by magic.
One other requirement would be for the circuit to be as small as possible, while still cost effective. The rest of the variables are not important for me, so, those willing to help me, feel free to do whatever you want about them.
There are no more details of the trick that are relevant to the circuit part. Well, it's not really a trick actually, it's more like something that attracts the audience's attention and makes them more interested. A gimmick.
P.S. If you have any more questions that you'd like to ask me in particular, for any reason whatsoever, feel free to drop me a mail (or IM) at my username at live dot com (giving my email address is okay, isn't it?).