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You can get 25mm for normal sockets, but easier with 35… sockets with usb require 35, and some smart light switches need more than 25.If the breaker is reduced to 16 or 20A, then the added cable can be 2.5mm2.
25mm box is for light switches. 35mm for 'normal' socket wiring and 47mm for anything involving 6mm2.
47 for grid switches