Both 40mm; one round profile and other square-ish octagon.
The Daytona measures 38.4mm at the bezel and the Nautilus has the largest dimension of 40mm (2 to 8 o clock position) at the bezel, yet still looks smaller than the already smaller 38.4mm Daytona. Both marketed as 40mm sizes on the websites.
Amazing how all the design changes make the watches look so much smaller or bigger than they really are!