There are really two main issues to consider - working space and storage space (specifically bulky storage like fabric and battings). I store all of my fabric and battings in the attic and have a rather large room for my actual work. 3-gallon Rubbermaid totes for zippers, interfacings, buttons, notions, elastic, etc are stacked in the sewing room, and I have a few of those rolling drawer carts for patterns and thread in the sewing room as well as a couple bookshelves. I also keep my computer in here. (actually two computers) and my treadmill. It's handy for hanging clothing on. ;) I have my loom in here, which I hope to learn to operate some day soon, and two file cabinets. I have a kitchen-type table for cutting and sewing. One wall is covered with a foam design board. The room has two large windows and a door leading to a little Juliet balcony. There are double doors that open into the master bedroom, too - I just love it!
What I don't like - carpeting. I wish I had a hard floor. all those pins...