AWhat size is the actual size (in inches) of Costco's full size sheet cake?

AI just Googled your question and found a couple of places that looked like it would have answers. But I would probably pick up the phone and call your local Costco and get an answer straight from the definitive source.

161.79mm x 265.43mm

ATwice the size of a half sheet

I just Googled it.  A half sheet cake is 12 X 16 so a full sheet cake would be 24 X 32 - the half serves 48 two inch by two inch servings so a full sheet cake would serve 96

I;m just quoting what I found on their website.  Home sheetpans and restaurant sheetpans are different.

More info about full sheets (I wrote this in another thread):

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.).

If this size pan doesn't fit in your oven, and you are baking two 12" x 18" (54 serving) half sheets, they would serve 108 total.

Call Costco.  I'm sure they would tell you.