If you are describing this person to the police (like if they were a suspect) you'd might say "average height, medium build". But if someone asked about a friend of yours "are they tall or short?" you could answer "medium height" or "average height". If someone asked about a friend "are they fat or thin?" you could answer "regular" or "healthy" or "average" or "just normal".

Hello, Neither fat nor thin could be slim or slender .

How do you call a person who is neither tall nor short? Average height middle-height How do you call a person who is neither fat nor thin? Healthy Average weight

Regular, Average, even "normal"