Well, no geese are going to 'put up a fight' against a predator and defend your ducks until you get out there. If a dog gets in the geese are going to be panicking and running away just as much as the ducks are, so if you're hoping for something to fight off predators then a livestock guardian dog is a better choice.
For something that will raise the alarm, then a pair of geese might be okay. However in a nighttime situation, when faced with a predator they might just remain silent, trying not to draw attention to themselves. So that might not work the way you want either. Mine alert at night when they see me, but then again they know I'm not a predator.
But, if you're okay with both those things and want to go forward, then Tufted Romans might work. They are the smallest of the domestic breeds; I even have a pair living with my Call ducks. They are great watch birds, and in fact they are the breed that saved Rome by alerting when the Gauls tried to invade, so they have a long history of sounding the alarm
Other good starter breeds would be Americans or maybe Sebastopols, both of which are also docile breeds and are good for beginners.
