Just random thoughts to your questions
First question: Height
What do you believe how tall the fairies are in a Midsummer Nights Dream?
The fairies can be whatever height their magical hearts desire. Titania is small enough to sleep in an acorn and big enough to wrap her arms around a man with the head of a donkey. Because fairies represent magic or dream than the ability to take on various forms and sizes seems a natural ability.
Second question: Changeling
Was the mother of the changeling boy a human or a fairy?
The mother of the boy was most certainly human. 'Votress' seems to imply priestess here. Many religious orders had, and still have, the priest and priestesses. Yes, there are priestesses.
Third question: Jealousy
Why do Oberon and Titania bless Theseus and Hippolytas marriage?
Here is where you must acknowledge the difference between humans and fairies. Humans have a short and fiery lifespan. Fairies have a seemingly indefinite life span. Therefore, how they deal with eternity and their own entertainment is quite beyond our imagining. Oberan loves Hippolyta and Titania loves Theseus. However, the fairies are wise enough to know that their love is fleeting and for the moment. Theseus and Hippolyta are human, mortal, therefore constantly dieing. Oberan and Titania have an immortal love for each other and for life. They adore nature and beautiful people and so they enjoy the nobles in our play. Oberan and Titania both go to bless the marriage because the wish the best in life, however fleeting it is, for those they love.