How would you modify this for an 11x15 sheet recipe or cupcakes?

Download and use one of many charts which show volumes of baking pans, then compare and bake accordingly.
For your question:
According to one from allrecipes.com the 9x13 cake holds 15c and would make one 11x15 sheet pan cake or 24-36 cupcakes.
According to one from joyofbaking.com the 9x13 cakes holds 14c and would make 1.4x sheet pan cake at 11x15 inches or about 28 cupcakes at 1/2c each. cooksinfo.com agrees about the 9x13 and sheet pans & has no info on cupcakes.

There's a lot of math here, but I think you double the recipe.
https://food52.com/blog/13239-how-to-make-your-baking-recipe-fit-your-pan-size