To avoid any problems. ask your electrician beforehand.
Cable size depends on the rating of the circuit breaker that will be used to protect it (probably 6 amps as this is the commonly available small size), the type and the route taken by the cable (if it runs in a bundle with others at any point along its route, if it passes through walls or thermal insulation, etc etc).
Your electrician will not only be testing your work, they'll be signing off that the materials you used and how you installed them comply with the rules at every point.
None of us know the "installation methods" that apply to your cable route or the measurements of the earth connections so while 1.5mm is a safer bet, we can't answer your question. Your sparky will be putting his name and his insurance behind his signature.