Wrap the chain around the large chainring and largest cassette cog (bypassing the derailleurs) with the flat edge facing away from the chainring and cassette. Add one inner link and one outer link where the chain starts to overlap. Find more detailed chain sizing information in the eTap AXS Systems Manual.
You can also see chain sizing in the eTap AXS System Installation video here: https://youtu.be/Bx-kB7rNFJQ?t=379