Ideally this should be a dedicated space, an entire room if possible or at least a sizable portion of one. You will need a clear space to work that has sufficient power sockets and a phone line connection nearby. Make sure your work space is clear from noise and distractions like a t.v. or washing machine.
Also think about the additional room you will need for other furniture you will need including a filing cabinet, stationery storage etc.
For more information about setting up your home office take a look at the VA Success Groups “Start Your Virtual Assistant Business Online Course” at: http://vasuccessgroup.co.uk/virtual-assistant-online-courses/start-your-virtual-assistant-business/