I've been asked to make a retirement cake that needs to serve 100+ people. They don't want tiers, so I'm looking at a sheet cake carved into a tombstone (morbid, I know, but the retiree made tombstones). How big should the cake be? I've read so many different things that I just want to make sure not to run out of cake!

AAre they serving alcohol? Is there a buffet? How much more + than 100?

A full sheet serves 100.

I would try to go with a  full sheet cake at least. If you are worried about not having enough servings, you can add another  1/2 sheet at the bottom and trim to size, or give them the option of having an extra cake that they can set back somewhere and use in case its needed. 

I say double as my customers tend to serve larger pieces. Everything IS bigger in Texas...even cake servings...lol. 

Also, keep in mind that a true full sheet cake is 18x24 or 18x26. Everyone kinda has their own idea what a 1/2, 1/2, and full sheet cakes are. 

1/4 Sheet = 9x13
1/3 Sheet = 11x15
1/2 Sheet = 12x18
Full Sheet = 16x24 (baked in 18x26 Bun pan with bakeable cardboard tray).

Additional full sheet info:
A commercial Bun pan is 18" x 26" (outside measurement), and because they are tapered for nesting or making them stackable, the inside measurement is 16.5" x 24.5".

A commercial full sheet is 16" x 24". They are baked in 16" x 24" bakeable cardboard trays that fit into the Bun pans (flat surface portion) which are used during baking for support and handling purposes.

A true commercial full sheet (16" x 24") serves 96 (unit wt. 106-124 oz.).
 

Thanks for the responses!

The party is taking place in the afternoon and will have a cold buffet (sandwhiches) and yes, alchohol will be served. I understand that not everyone will have a piece of cake, but since I'm sure that they will be cutting party-size pieces (as opposed to wedding-size) I would hate for them to run out!