1. Mount the image
2. Delete the files you don't need.
3. Dismount image.
4. From the terminal : hdiutil resize -size xx{m or g} /PATH/TO/DISKIMAGE.dmg
xx= the size you want the image to be.
Use either M for MG or G for GB

i.e. hdiutil resize -size 25g ~/Desktop/mystuff.dmg