Normally such dimmers have a 300w maximum capacity. Somewhere on the dimmer it should tell you it's capacity. If it doesn't, spend the lousy 20 bucks and go to Home Depot and buy a new one -- perhaps even a 600watt model.
Be advised, your ARRI will be pushing the 300w dimmer to it's limit ....with no allowance.
The Router Speed Control from Harbor Freight does a nice job as a 1K dimmer for $13. It seems to work just as well as the $120 one I bought. Same amount of electrical buzz at the lower end, but that is to be expected.