No one can tell you the file size because JPG compression will be different for every photo. Pictures of thousands of blades of grass will take up more space than a close up of a face. Generally they'll be in the 2 - 4 MB range, I think. But that's just a rough estimate.
Looking at a series of photos I have taken they vary from 1.7mb - 3.6mb.
Night time grainy photos will be the largest. As stated above complicated detailed photos compress worse so the files are largest. The grain/noise in low light photos is "detail" as far as the compression goes.