It was asked to me this question of how large the file size is with different size camera's and cards, I dug around and came up with this chart.

From what I can surmise there is no rule of thumb or formula you can use to figure it out, It depends on many factors, What type of file's, JPEG, RAW, TIF and there differences in what types of photo's you take, is it light or dark, is there allot of action in it or is it mostly blue sky?

Assuming you have taken average photo's and your camera saves them as a standard JPEG you can use this chart to see and compare the different mega pixel sizes to the average file size of the photo's taken.