Can I use a 16/3 extension cord for a refrigerator?
Wire gauge tables for 16AWG list the maximum carrying current from 4 to 10 amps continuous; for 18AWG, 2 to 5 amps (for under 50-ft lengths, at least).  I think it's unlikely that your fridge draws more than 2 amps under any conditions ( if you have a tag on the unit which lists watts, just divide watts by 120 (voltage) to get amps), so any extension cord you have will be fine.  However, if the fridge has a 3-prong plug, be sure to use a matching 3-wire extension cord for grounding safety.