I have my first big wedding shower coming up. They need enough cake for 100 people and want sheet cakes. Should I make 2 or 3 half sheets? I quoted her the price for 2 and because the Wilton party guide shows the 11x15 should make 60 slices. But now I'm getting afraid that they might slice it bigger and run out. What do you recommend?

The Wilton guide for 11x15 is for a 2 layer cake, and even then I doubt it would feed 60 people, unless it was cut really small. I think I would have go to with 2-12x18 sheet cakes.

If you want make them 12x18 instead and you will have enough. And two would be enough.

I don't have the larger pan yet. Should I make 4 of the 11x15?

No you will have enough. I check my serving sizes. I don't think you would need even three cakes doubled. I have found at parties not every one will eat cake. Diets play a big factor at gatherings. You should have more than enough.