Depends on who you talk to; according to Magna Carta, the island of Britain is just the mainland while Great Britain is the mainland plus the many smaller islands around it like Mann and Sheppey. But then according to Roman texts, Great Britain is the entirety of the island minus the outlying islands while Ireland is basically called Little Britain (Britannia Major/Minor respectively).
It depends. Generally we think of Great Britain as being the entirety of the mainland UK.