2880 is the max.
I am experiencing a similar problem. I have hundreds of symbols, most of which export just fine. A few symbols, however, do not. Although these symbols are typically modest in size, 1x49 pixels for example, Flash throws the error "The bitmap is too large. The largest bitmap that can be created is 8000x8000 pixels." which halts the script until it is acknowledged by the user.
It seems that this "too large" error might appear anytime the width or height is out of range, as in it is either too large OR too small. If you allow the Export PNG dialog box to appear (or manually do a File...Export Image...), it has a Width of 4 pixels. When I try to correct the value to 1 pixel, another error is shown saying "Please enter a number between 4 and 8000." Anything outside this range will not be accepted.
I also noticed that when I specified "false, true" for the optional 2nd and third parameters, the Export PNG dialog fails to appear.
I am using Flash Pro CS5 on a PC.
I tried this using Flash CS4/PC. I had an even worse time with the dialog, as I changed the width and height, it automatically changed the dpi from 72 to whatever value entered.
So in order for the script to be able to exportPNG on all of the symbols in the library, I have to first check the symbol's dimensions, and either abort the export operation, or correct the values, forcing a minimum dimension of 4 pixels.
Same problem here. I'm also using xJSFL, but that shouldn't be a problem. I get the same "8000x8000" error. I'm using Flash Pro CS6
There is absolutely NO way to export PNGs with a custom size because of this bug. Setting the document size before exporting doesn't help. It is always exported with the default document size.
One hint: The symbols I try to export are created via JSFL. Maybe this is a problem?