Right, assumming you really mean an electric shower (a water heater) as opposed to a power shower (a pump) ...
A rough guide is that 6mm is good for upto 8.5kw most of the time, but depending on how it is installed, you mileage may vary*...
I recommend installing 10mm if at all possible, future proofing, even if you only want a small shower atm.
Also, are you familiar with the requirements of part P of the building regulations?
*Consider: Installation method, factors for: grouping, thermal insulation and ambient temp also voltage drop and Z's at furthest point (and going of an a tangent... make sure your bathroom is supp bonded)
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