When a slip roll such as this one:
http://www.harborfreight.com/cpi/cta...emnumber=36698
is spec'ed as 12" x 20ga, I assume that means it can roll a 12" piece of
20ga (doh). But I also assume that if the piece is less than 12" wide,
it can be more than 20ga thick. The question is: how much thicker? Is
there a rule of thumb for the relationship of thickness & width?

I assume (again) that the limiting factor is the diameter of the roll.
That is, for example, if a 12" piece thicker than 20ga were put through
this roll, the rolls would deflect & cause uneven rolling of the stock.
But narrower pieces would be rolled near one end, minimizing the
deflection of the roll. ??

Thanks,
Bob