I think 3/4" is the max for that particular vendor of flow sensor. If you are comfortable with Arduino's you can save by just building what you need using an AVR instead of a full blown Arduino.
Are you trying to automate the water change process? I'm not sure I'd trust a flow meter to that task, a level sensor or float would be a better bet. If you just want indication, why not look into one of the pool/spa flow indicators that can be installed in PVC pipe? Blue-White makes some nice looking ones I was thinking about putting on my sump return for giggles.