ispell-extra-args

ispell-extra-args is a variable defined in `ispell.el'.
Its value is

nil

Documentation:
If non-nil, a list of extra switches to pass to the Ispell program.
For example, ("-W" "3") to cause it to accept all 1-3 character
words as correct. See also `ispell-dictionary-alist', which may be used
for language-specific arguments.

You can customize this variable.