Don't worry, I do this all the time! I'm always making huge VMs and then wanting to shrink them later. :)
For starters you will need to resize the partition inside of Mint. Follow this excellent guide for resizing ext3:
You do not need to run this command:
tune2fs -O ^has_journal /dev/sda1;
resize2fs now has full support for ext4 so you're good to go.
After you have resized the guest partition, shut down the VM. Attach a new disk to the VM with the smaller size you want. Boot the VM with a Linux boot CD, I'm a big fan of "Trinity Rescue Kit 3.4"
In Trinity lets assume you have the following:
/dev/hda (good large disk)
/dev/hdb (empty small disk)
At this point
hda will have a smaller partition because you followed the guide above
fdisk -l to list all known partitions. Then run
fdisk /dev/hdb and create a partition table identical to
dd if=/dev/hda1 of=/dev/hdb1 for every partition that you have.
Now the final stage is to install the boot loader. Follow the guide here:
After you are done, shut down the VM. Eject the live CD ISO. Detach but don't delete the large disk. Click
properties for the smaller disk, and choose the bottom menu item on the left. For
device position, change it from
0. Boot the VM and make sure that all your data is there. For a final check you can run
sudo touch /forcefsck; sudo shutdown -r now which will reboot the VM with a forced disk check at startup. If everything is ok, delete your large disk image.
If you have any problems with these steps, let me know. After you are done you should feel like a boss (cuz you are!).