Early on we chose two packet sizes for PingER. These were and still are 100 and 1000 Bytes.
We chose two since with yesterday's slow network we had hoped to be able to deduce the speed of the connection (bit/s) using the different packet sizes and associated RTTs. This did not work well then and even worse as with today's fast networks the RTT depends very little on packet size.
We did find the two packet sizes useful in a few cases where the connection only exhibited problems with large packets.
For the large packet size we wanted to stay well away from the maximum limit in case the payload was reduced by other emerging requirements such as MPLS.
The choice of 100 and 100 was made as being easy to remember.
It is possible to configure a remote host so it is only pinged with the small packets thus saving bandwidth.