When a picture is saved in a certain dpi (dots per inch), you basically get what you saved...The picture "quality" can't really be improved much..On the other hand, if you mean that the "viewed picture" on the screen is too small, then you should be able to view it larger in a number of different viewers. On Windows XP, the default "Windows Picture and FAX Viewer" allows the picture to be "zoomed". In addition, Irfanview, a nice free viewer at the link below allows you to zoom on pictures or save them as a different file size.
Hope this helps.
I created a photoshop document but when I open it the size has diminished significantly from the picture I saved. Can I enlargen this thing?