I have one room dedicated just for my MS, Merchandising, and self publishing business.  It contains three large file cabinets, three large bookcases, and a computer center that wraps one entire wall and part of another.  
Year before last I was doing my own taxes, but decided to let a lady from H&R Block go over them to see if anything jumped out at her.  When she returned my taxes she came in my room and said, "You definitely have a home office.  Nothing else could go in here."  I did not mention that a couple of my merchandising jobs require me to receive large shipments and keep them until the date I install them in the stores.
I now have a LLC set up because I added something else in the mix, so I had to get an EIN.  Some of our MS employers are starting to ask for them anyway.
The last two years I had a CPA doing my taxes because they were starting to get complicated.  He advised instead of deducting for a home office that he just spread the same expensed under different headings.  As long as he is satisfied, and he would have to go with me if I am audited, then I am satisfied.