Making stars smaller
This is a way to make a large number of field stars smaller, it uses the Minimum filter in Photoshop.
Using the Magic Wand selector tool and the tool set to sample size 3 x 3; Anti-Alias on; Contiguous on and sample all layers, click on a part of the background, well away from stars. Try repeatedly till you get all the background selected, shown by the stars appearing to be selected. The Marching Ants will also be seen all round the photograph. Then click on Select/Inverse. This will select the stars and should also remove the Marching Ants from the outside of the whole photograph.
Click on Select/Modify/Expand and do so by about 2 pixels.
(Take this opportunity to click on Image/Adjustments/Hue and Saturation and brighten up the stars with some colour, don’t do too much.)
Click on Control-H. This will hide the Marching Ants so you can see the next actions without the Ants spoiling things. Clicking on Control-H again will bring them back.
Click on Filter/Other/Minimum and when the Panel screens, set it to 1. It will probably already be set at 1.
Click on the small window in the Panel on and off to see the effect, and also the Preview button to see the effect on the whole photograph. Click OK.
Then immediately click on Edit/Fade Minimum. This will bring up a small Panel with a slider. Leave the Mode set at Normal, move the slider to a position that suits your needs, and click OK.
Click on Control-H to bring back the Marching Ants or Control-D to remove the Selection.