If you can get or have the engine apart the best bet is to borrow a micrometer and measure on. At a guess .014, .015, or .016 music wire.  .35, .4, 0r .45mm.

The wrist pin should hit the retainer just barely above the center of the wire.  It should be nearly impossible to push the retainer out of the hole with the pin by hand.  If you can push the wrist pin through by hand it certainly will come out when the engine is run.