Explain why the size of your muscles is partly an acquired trait, and partly dependent on dna.

Your DNA determines how many muscle fibres a person is born with. This number never changes cause one can't make more muscle fibres during their lifetime. With more muscle fibres, one has the potential to be stronger.
It is an acquired trait because you could train your muscles to become bigger. When you train, for example resistance training, your muscle produces more myofilaments which increases their diameter. This is why you develop large muscles when you train  

ypc