Internal helper function for checking specific arguments used in custom_search.
check_args_size(size, negate)An integer specifying the size (in number of characters) of message to be used as search criterion.
If TRUE, negates the search and seeks for
"NOT search_criterion". Default is FALSE.
NULL if arguments are correct.
Other customsearch checkargs functions: check_args_date,
check_args_flag,
check_args_string,
check_args_within