You are taking the first four bytes out of the image and converting it to an IEEE floating point. I'm not an expert on image files so I'm not sure if the first four bytes are always the length, even if this is the case it would still not be correct (see the specification). However the length of the file is already known through the length of the array, so an easier way to get the size is:
byte[] imgData = prod.ImageData;
float mb = (imgData.Length / 1024f) / 1024f;
To address your concerns: this will still work for large files, consider a 24TB example.
var bytes = 24L * 1024 * 1024 * 1024 * 1024;
var inMb = (bytes / 1024.0F / 1024.0F);