http://www.keeneymfg.com/cad_drawings/41-P-Traps has various dimensional drawings. To keep things smaller, they can us a 1-1/4 instead of a 1-1/2 inch trap. Add a bit of space for the tail piece. I would think it would vary how flat=bottomed the sink bottom is as to how much tailpiece would be needed. Will you need to leave clearance to remove and insert the trap? Or can you make the compartment bottom lift out for easier maintenance.