3D Copy Would like to get output to size under 25GB can it be done?

[dgrams]

I'm wanting to start compressing my BluRay movies down to like 12GB in size (for main movie) Instead of just lowering the size down to 25GB.

It works better for playback on my WD TV Live
when the .iso images are a bit smaller in size

How Can I compress them down to a custom size like this?

(For either normal BluRays or for creating 3d backups into SBS .iso images)