This comes from the RFC 1242 and RFC 2544 which is referenced in the RFC 3511.
In RFC 2544 9.1 it says:
9.1 Frame sizes to be used on Ethernet
64, 128, 256, 512, 1024, 1280, 1518
These sizes include the maximum and minimum frame sizes permitted by
the Ethernet standard and a selection of sizes between these extremes
with a finer granularity for the smaller frame sizes and higher frame