advise on building a storage shed

I have never grown before but, i have a friend who is going to give me a hand to get started. I am going to build a shed in my backyard. I was thinking of a 10' x 12' and 10' tall. I want to start with 12 in veg and 12 in flower. later i might want to expand. is this over kill on the size? is it to tall? I was thinking of a barn style roof or should i keep it a 'A' frame? any advise would help. any one built something like this?