One method would be to go to your local DYI and get a 3' length of 1/8" rod. Then place this rod through one spoke hole and into the opposite hole. Mark the rim's nipple beds on this rod, remove and measure this length along the rod. Repeat at least two more points around the rim.
Or you can google the rims and you get this link which claims 602mm ERD. This only took me about 1 minute. Andy.
https://leonard.io/edd/rim/277-araya-adx-2w-700c