Add ENum Attribute to existing table using Alter

I am using oracle XE and trying to add a attribute that restricts to only three available value 'Small','Medium','large' to an existing table using Alter Table and Enum. Tried doing, ALTER TABLE TRUCK ADD TTYPE ENUM('SMALL','MEDIUM','LARGE'); but gets invalid option ALTER TABLE TRUCK ADD TTYPE ENUM('SMALL','MEDIUM','LARGE'); * where the error highlights after ENUM. I think I am having syntax error. Please help to resolve.