Listen to what JL says...typically manufacturers advertise the smallest possible box size. If you go smaller, you will get a peaky box that will sacrifice low end extension, and that's bad.
The smallest I would go is 1.25.
One of the tradeoffs of choosing their entrylevel sub is that you give up power handling and you need a bigger box.
Don't worry though, the sub sounds nice (not like many entry level subs), my friend has one and his is in 1.5.