Wouldn't it be possible just by doing the math?
Each Boolean/Char (in a string) = 1 byte, Each short = 4 bytes, Each Int/Float = 8 bytes, Each Long/Double = 16 bytes, Each ARGB Pixel (in an image) = 4 bytes*
*Note: The image size would probably be smaller, due to compression. For a rough estimate, if you know the compression rate, just do the math.
I asked the question beceause I have no idea of the overhead. My maps tend to be several thousand in records and they contain documents. Java has abilities to do this, but I have no idea on how objective-c does this, if at all.