Standard only says that it is implementation defined:
35) Subclause 6.1, “<data type>”:
i) The maximum lengths for character string types and binary string types are implementation-defined.
The max varchar length was set based on MySQL's max length (32K), thought it made sense to do the same for char. We can make the char limit a bit bigger, but if users want very large string values then they probably should be using string columns. Any preferences on size?