How to use the METHOD_OPT parameter in DBMS_STATS.gather_table_stats
Looking at the syntax I would expect that COLUMS size_clause is a global setting, that can be overridden by specific size_clause for a single column. So in case of FOR COLUMNS SIZE 5 col1 SIZE 6, col2 means SIZE 6 takes precedence for col1, but col2 falls back to the default SIZE 5. This is what I would certainly expect, but I haven't verified this experimentally.