I read a startling research item by Mashable that may not have been apparent to everyone. It wasn't apparent to all of us Autodeskers who have a long tradition with Beer Busts at special events and Autodesk University. There is about 28 more square inches of pizza in the single 18” pizza (excluding the great stuffed crust debate). Yes, on a normal thin crust pizza you get more ordering the 18 inch pizza instead of 2 – 12 inch pizzas.

Fermat's Library. https://twitter.com/fermatslibrary/status/1082273172114862083

In verifying this fact because I don't always blindly trust Mashable, I'm giving you a handy AutoCAD tip on use the awesome under appreciated FIELD object. You can show the live value of an object, even an associate hatch to show the area.

Open AutoCAD
Draw a circle using command C
Use BHATCH command to place a boundary hatch on the inside of the circle.
Now use the FIELD command and select > Object, then select the BHATCH and place the value. Now when the boundary of the hatch changes, so does the value. This can be used to show area of floorspace, landscape, steel, roof, parking lot, or pizza.

Here is an oldie but a goodie blog post I wrote showing a more detailed tutorial on using AutoCAD Fields.
StarBacks BBQ Floor Space Planning with AutoCAD FIELDS

Cheers,
-Shaan

Update 1/9 for the stuffed crust pizza lovers.  Pick the two twelve inch pizzas since you will get more linear crust.

18" Pizza
Circumference   56.5487
Area  254.4690

12" Pizza
Circumference   37.6991 (times 2 pizzas)
Area  113.0973

Total linear stuffed crust 75.3982 vs 56.5487