I used to write in Normal Size/Sans Serif while replying to emails. It works fine for me. But recently, I started to adapt my font/size based on the sender email style. If someone is using a Serif font, I reply in Serif. If the font size is large, I increase the size.
I think there is a reason people prefer one font/size over another. For example, someone might write in large size because it is more readable. So, if he sent an email with a large font size and replied with a small one, he might find my reply hard to read.
Someone might prefer Serif over Sans Serif. So, it could be a different taste. By adapting my writing, I think I'm adding more harmony to the email, so we are writing with a similar style—no switching between reading and writing in different fonts or sizes.