An electric geyser is rated as 1000w, 220 v marked on it what should be the required minimum rating

Watts equal volts times amps, so 5 amps x 220 volts = 1100 watts. However, many devices draw more than normal to start, so you should use at least a 15 amp circuit. It is dangerous to change fuse size without an electrician's recommendation. It depends on the size of wiring and nature of service connection. A 15 amp fuse is normal for most household circuits.