Boy, I'll say that's plenty of cake! The wilton chart tallies this to be 178 servings!!!!
I do understand you will lose some in the carving.
Do you charge more per serving when you sell party-size servings? Or are you just throwing in free cake?
Standard servings: 1x2x4 = 8 cubic inches.
Larger serving: 1.5x2x4 = 12 cubic inches. 25% more per serving.
So if your standard serving price is $3/serving, if you're giving them 25% more cake in the serving, then your price should reflect that and the price should be $3 x 1.25 = $3.75/serving.
Here's how I cut a cake for the wilton wedding chart: http://cateritsimple.com/_wsn/page10.html
Notice the pieces of cut cake ... they are not teeny-tiny. I've cut cakes this way at family birthday parties and I watch the reaction .... it's perfect for little kids, and the adults comment on how it's a nice piece of cake, not too big and not too small (because after they're done eating, I survey them!).
If you're giving away 75% more cake than they need, please be sure you are charging appropriately for it.