Ok I assume, i am sure you will correct me if i am wrong, that you have the 11 inch mac book air as do I. Now i could have bought a bigger version but didnt as the 11 fits where I need it I agree that the size of text/window thing for mail is not brilliant but could be worse, I have mail open in full screen and use the 4 finger swipe to move between open Apps as i find that this helps a little with my failing old eyes when reading mail. I then do the CMd + thing if i am still struggling. I agree that you cannot enlarge the reply window and this can be testing sometimes particularly in low light for me however I have found that by customising the display settings ( system preference/display/ colour) and then calibrating, it helps for light conditions. This does not affect the resoution.
Of course if none of the above helps me i use the big fuck off I Mac but i rarely need to
Nothing in this life is perfect but some products are actually better than others and Mac is one of those.
