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.