Minimum Frame Size
Speed of light is about 3x10^8 m/s in vacuum and about 2x10^8 in copper
So, max Ethernet signal prop time is about 12.5 msec, or 25msec RTT
With repeaters, etc. 802.3 requires 51usec, corresponding to 512 bit-times
Thus, minimum frame size is 512 bits (64 bytes); also called slot time