I need to install a subpanel for some circuits in a newly finished basement.  The main panel (200amp) is short on spaces and I can’t use the twin breakers (Seimens panel).  The subpanel will be right below the main panel.  I was going to use either a 50amp or 60amp breaker to power the sub.  What gauge wire is needed for such a short run?  I would use #6 THHN in a conduit but is #8 okay for such a short run?  I have done subs before so I know about the insulated (or isolated) ground requirement.  I would use the #6 wire for any sort of longer run but is it overkill for a 1-2ft length? 

Thanks for the help.