What exactly do you mean by size ?
The size of the file in MB ? I would consider anything up to 5 MB an average high resolution photograph
The size of the file in pixcels ? Again, anything around 4000+/- x 3000+/- would be an average high resolution image
The apparent size of the image in an email ? This is totally irrelevant to the size you might want to print it at or save it at.
The first thing to do is to download the image anyway, then look at the size when you go to print it and first look at it in print preview. To make the image smaller at this point, increase the printing resolution in terms of dots or pixcels per inch/centimetre (conversely, to make the printed image bigger, reduce the printing resolution).
If you wish to save the file but as a smaller file size or lower resolution follow the steps outlined by FreeBooter. Or save the original then make a copy at the lower resolution. That way you have the original high resolution image and a copy.