Maximum size of a column cannot be reduced

Maximum size of a column cannot be reduced

I am trying to update a form via the Form Definition page, but I get the following error message when I try to save the script


 Error modifying the page
Maximum size of a column cannot be reduced, when the table is not empty -- tableName : t_116668000000015301 -- columnName : City_116668000000015428

I am not even trying to modify the "City" column.

How can I resolve this?