Daniel M. Ingram:
Mine have varied in size, depending on the jhana and what object I was using.
They have tended to arise when I was actually using a Kasina object, though have seen plenty of brief bright white lights and jewel-tone sparkles and other similar things as well, usually lasting no more than seconds.
However, when kasina objects (candle flames, dots on computer screens, pentagrams, plates, the moon, etc.) have been used and the kasina afterimage is then the object, and then the nimitta emerges out of that, it has tended to be rather small (say the size of a pencil eraser if held at arm's length) such as when the candle flame afterimage turns into the burning red clean, pure circle, and then tends to be about 2-3 times as wide (say a nickel held at arm's length) when it becomes the spinning yellow star in the red circle with the green and purple rings around it, then becomes significantly larger when it becomes the black spot (say the size of tennis ball held at arm's length or maybe even as big as a softball held at arm's length), then it becomes much larger when the lines around that black spot start forming and swirling slowly around it, like pale golden-white tangents to it, say the size of a pizza-baking pan held at arm's length as it expands to be the many-radially-symmetrical aztec-patterned rainbow flux lines, then fills the whole field as it becomes things like a black hole drawn Cosmos-style in those same rainbow flux lines, or whatever, filling everything.
Helpful?
D
A "kasina nimitta" is very different from the regular nimitta, since the former will be there from the start, while the latter arises as a sign of concentration. The drawbacks with kasinas are that you need a physical object (not a big deal), it takes time to "charge" before the session and, most important, that it might fade before you get to the jhana or before you get a concentration nimitta to use in its place. That said, I'm not so much into kasina meditation, and don't have the complete picture.
A regular nimitta lasts as long as you keep up the concentration, and from access or earlier and onwards it will potentially be there all the time, looking at you face-to-face, like a little companion. The risk with extravagant and detailed descriptions of nimittas is that the beginner will decrease the chances of spotting it. I made use of nimittas a couple of years before I read a description about it, but still it took a while before I realized what the author was talking about. The regular nimitta is not very exciting. Visudhimagga (I think it was) describes the counterpart sign (patibhaga-nimitta) as "cotton white", but I would say it is more like white cotton dipped in dark blue paint with a hint of grey... but if you look for something resembling that description you might not realize that you are already staring at it. It's subtle to the untrained eye.