To me "Kepalanya cukup kecil" means "His/her head is small enough". So, may be "His head is pretty small" could be translated as "Kepalanya agak kecil"?
"Enough" is not the only meaning of "cukup". According to my dictionary, "cukup" also means "quite". So, the English sentence "his/her head is pretty small" is correct.
Pretty doesn't mean enough! It is a weak intensifier.
I don't understand this translation. See other comments!
'Pretty small' would not mean 'small enough' in English. I could be used rather to signify 'quite small' (relative to another head perhaps).
If a medical person says that, its not pretty.
I agree with donny309620