Today's media and print specifications

Most print material whether it be offset litho or digital print requires your photos to be 300 dpi or above. Large prints such as posters and billboards will normally be printed at a resolution of 150 dpi (the final result will be viewed at a greater viewing distance so detail does not need to be so intense). Please check with your print department before selecting. This will mean that your A3 file at 300 dpi when converted to a file size of 150 dpi, will give you a file size of 200% enlargement, so the final file size will be A2.
Please note that this conversion and/or reducing the size will not degrade your image but if you enlarge the image size and keep the final output size at 300 dpi then this enlargement may yield some image loss.

Conversion chart to aid your image size selection