Ok, that'd be the issue then. Changing a fields type doesn't clear previous parameters and the core validation check looks for the length parameters since that function didn't take into account the later added option of changing a fields type. Easiest way to remove that validation is change the field back to text, empty those parameters, save, then change it back to select.
forge.joomlapolis.com/issues/7431
Added a bug ticket to look into a means of fixing this when a fields type is changed, but you'll still need to perform the above steps to fix your field.