적당하다 is more commonly used for physical suitableness like size, volume, height, weight, temperature 적절하다 is more commonly used for conceptual suitableness like usage, expression, speech, timing, position, action e.g. 크기가 적당하다 무게가 적당하다 표현이 적절하다 그는 야구 감독으로 적절하다.

Hi 적절하다 is perfect same, 적정하다 is almost same but not perfect. thank you.