A free-standing booth should be about 7 to 8 feet tall, 6 feet wide and 6 feet deep. If you're building into an existing room as opposed to a free-standing booth, try a closet or an existing corner. A full room isn't a good option in this scenario because it will be too big to achieve the proper acoustics.
One critical aspect of a vocal booth is soundproofing. Failure to soundproof an audio booth will result in recordings that are little better than recording in an ordinary room. The floor should be covered in a low-lying carpet. The walls and ceiling must be covered with a sound proofing material such as foam, then built over with secondary walls, flooring and ceiling. The purpose of placing secondary structures over the soundproofing material is to prevent acoustics from leaking outside of the booth.
A ventilation fan is important to maintain proper air quality. A hot, stale environment caused by an insulated room can be unsafe. You can find a quietly operating fan that should keep the noise in the booth to an absolute minimum. Virtually noiseless, small fans are available for purchase.
Don't incorporate a window into your booth. The purpose of the booth is isolation and sound-proofing. A window allows the sound to leak as well as render any type of isolation useless. Consider a two-way video feed instead. This will allow communication between the booth and the workstation with no unnecessary escaping or inclusion of noise. When installing a door, use a pre-hung exterior door. These doors are made with a quality weather stripping already and require little to no additional work for sound-proofing. If you find you're not getting the quality you want from your door and you need extra sealing, try a Neoprene stripping. This will allow you to seal the door without having to construct a new door.