Very popular size of junction box. You get the most bang for your bucks with the five outlet box.
It's a sensible choice because it future proofs your project - just watch how fast the outlets are used up:
1) Pond Pump
3) Pond Lights
4) Fountain Display
5) Air Pump
This just gives you an idea of what you might need.
Other electrical possibilities include blanketweed control devices, pest deterrents, additional garden lighting etc.
Enclosure dimensions: 150x110x70mm
Supply voltage: 220 to 240 Vac 50-60 Hz
VA per outlet: 1.5 amps 360 watts
VA total:5 amps 1200 watts
Fuse type and rating: T5 Amp Anti-Surge 20x5mm
Environment: temp. range -10 to +50 Deg. C
Sealed to IP44