This list gives you DPI and image size in pixels for standard ISO paper sizes.
Generally you are OK to print @ 150 dpi but sometimes sharper output is required for more detailed work. You can use these in Photoshop or directly from SU when playing mixed media and presentations.
 

A4 @ 300 dpi  -  3507x2480 pix

A4 @  200 dpi -  2338 x 1653 pix

A4 @ 150 dpi  -  1753x1240 pix

A4 @   72 dpi  -    841x595   pix

 

 

A3 @ 300  dpi  -  4960x3507 pix

A3 @ 200  dpi  -  3307x2338 pix

A3 @ 150  dpi  -  2480x1753 pix

A3 @  72   dpi  -  1190x841   pix

 

 

A2 @ 300 dpi -  7015x4960 pix

A2 @  200dpi  - 4677 x 3307 pix

A2 @ 150 dpi -  3507x2480 pix

A2 @ 72   dpi -  1683x1190 pix

 

 

A1 @ 300 dpi -  9933 x 7015 pix

A1 @ 200 dpi -  6622 x 4477 pix

A1 @ 150 dpi-   4966 x 3507 pix

A1 @   72 dpi-   2383 x 1683 pix

 

 

 

A3 comparative size of the image

 

@72    dpi  = 2932  kb

@150  dpi  = 12736 kb

@300  dpi  = 50961 kb


These figures are taken from the Max utility and are universal So check the file size increase. When you double your DPI it quadruples number of pixels.