What words or expressions could be used to say someone is/looks handsome? Either words or expressions, formal ones or informal ones.
These are some idioms which may be an answer to the question:

眉清目秀 (mei qing mu xiu) This literally means: "(His/this Man's) eyebrows are clear and (His) eyes are beautiful."

玉树临风 (yu shu lin feng) This literally means: "(Men are/He is like a) grown jade tree which has faced the wind." 

英姿挺拔 (ying zi ting ba) This literally means: "The handsome appearance (of Him/this Man) is outstanding." 

帅气逼人 (shuai qi bi ren) This literally means: "The handsome attitude (of Him/this Man) is daunting."

Chinese idioms, usually, only consist of four characters. And all of the above given examples can be used to refer to a handsome man. 

Be aware, though, that these are formal expressions, so, they are literary and are likely to be interpreted as being corny when used in an informal context.