You can upholster the final chair like I did or just paint it and fill it with pillows. The plans for this instructable are for a 4' diameter chair that will (just barely) fit through a 30" doorway. You could scale it up or down easily depending on the size of person it's for and how much "surround" you want.
The chair stands on the base by gravity alone and the unattached design allows for easy readjustment of the ball. I like sitting in it more upright, and my daughter likes it kicked farther back.
I made this for my daughter's 13th birthday and it was a big hit. She's an avid reader and it makes a perfect cozy zone to open a book in. When she saw it she called me a "mad genius" which is high praise from a 13 year old. One of her first questions was "how will I get it to college?" I think she likes it and plans to have it around for a long time. It was a big project, but well worth the effort.