"Can i convert this resolution (0-256) to a length scale (meters)? "
Sure, but you have to have an independent definition of what full scale is in actual distance--it can't be obtained without that external bit of information from somewhere.
Or, alternatively, if you can identify a particular location in the image and know what its elevation is, you can back out the scaling from those two pieces of information. Clearly, this is going to be much more susceptible to estimation errors.