Here is a map of Zanzibar beaches covering the most important spots. Zanzibar is app. 120 km long and 60 km wide. It is easy to transport yourself with local bus, taxi or car.
The roads going North-South and West-East are wide but due to mixed traffic ( cows, bikes, moped, donkeys, ox carriages) time to spend going from North to South is 2-2,5 hours and West to East 1,15 h.
Most things to do and explore is on the South East coast.