Strictly speaking you should go up to 10mm squared as typical 6mm house wiring (T&E) has a recommended maximum load capacity (according to IEE 16th ed.) of somewhere between 38 and 43 amps depending on conditions. This is a safe figure, based on continuous current flow. A 10.5kW shower at 230V runs at about 45 amps, for about 10 mins each time.
Secretspirit's answer is totally incorrect. A trip switch only detects earth leakage, but the shower must be connected to one.
Also, a power shower only uses a small current to operate the pump and does not heat the water.