I will double check but I think 1 1/4. Let seem if a hole saw will work as thinking I may have to use an existing hole.
What size of wiring are you running ? 11/4" is a pretty big hole, that would fit a 1" conduit fitting. Most existing box knock outs will be for 1/2" and 3/4 " conduit. You will need a knock out set to expand them to 1 1/4".