I cant really give much advice on size as im always worrying if my 2 have engough room!
As other people have said, if you can bunny proof and have your new friend free ranging then that is the ideal set up for them, providing that you doint mind things being damaged/destroyed when your not around to supervise.
Its for this reason that i am getting a pen for my 2. Its a puppy pen with 8 panels that i can have as a 4x4 or 6x2. Ive bought an extra 2 panels so that i can extend it to 6x4 if i like, but i havent really decided yet.
If you want some ideas on set up, there are always pictures in the storys and photos section and the housing section.
Apart from that i cant really provid much advice, but what everyone else has said is really good advice. Good luck with your new bunny :wave: