Micrometer
If you don't have one get a 5mm drill bit and drill into a spare piece of wood and try to fit the switch into the hole, if it doesn't fit go up 1/2 mm increments, and if the hole is too big (Which i doubt) go down by 1/2 mm increments.
Most DPDT switches are in the 5-6 1/2 mm range.
BTW Sorry i don't know how to convert from metric to imperial.
