troubleshooting creating an image file of drive smaller than actual size

So I've managed to successfully execute the program but I've run into another problem. The image tries to duplicate the exact size of the volume.

I have a 1.5TB drive and I know the data on there is significantly less than the full drive, say 70GB. I also noted there is a command "disk-size int" within the program. Is there a way that I can claim that the drive is 80 GB and try to create a decrypted image file that way? How would I write that out?

Would this also work if I know that there is roughly that much space on the drive? or will some of my files be damaged?