The limit so far seems to be 2000-2500 pixels (at 300dpi). And it can vary depending on the kind of flame, I could render images using a certain setting and this same setting didn't work for other flames (with the same number of layers). I'm rendering all the flames at 150 dpi then if possible I'm resampling them in Photoshop to enlarge them a little more. For "blurred" images, it's possible to resample them without changing them too much or adding extra artifacts. I have 512Mb and the memory goes to 0 when rendering flames very often.