Antenna size used in wireless morse communication
As your quote says "500KHz has been an international calling and distress frequency for Morse code maritime communication".  This frequency was used by large ships, so antenna length was no problem. Morse code (on any frequency) has not been used for marine communications for many years.

Morse code can be used on any frequency - Amateur Radio operators still use it on many bands in the short wave spectrum - bands from 1.8 MHz (160 metres) to 28 MHz (10 metres).