How to determine a capacitor value across a switch carrying DC
This page has several suggestions to provide arc suppression for pushbutton (or relay) contacts controlling a load.  

This and other pages I looked at all use a resistor in series with the capacitor to limit inrush current.  The examples are shown for inductive loads, but I would think the RC solution (known as a snubber) would work for restrictive loads as well (whereas the diode suppression across a relay coil is specifically designed for inductive loads).



The suggested values of R and C from the page I linked to above are 0.5Ω to 1Ω per 1V of contact voltage, and 0.5 µF to 1 µF per 1A of contact current, although I have also seen values of 100 Ω and 100 nF used.