Hello, I have a customer who needs to feed about 70 people and she'd like a sheet cake. Would two half sheets be enough? How can I smoothly frost a sheet cake this big? I've tried searching for tutorials online and haven't found anything on a sheet cake that big.

According to Wilton's chart a 11x15 sheet cake will provide 74 wedding size servings which are 1"x2".  So one half sheet cake would be enough for your customer.  Just make sure you let your customer know about the size servings and maybe even provide a cutting guide for their reference.

I’ve made many   Several things to consider  1) need an extra strong board  2) make sure it will fit through all doors/openings   3) consider making it 4” tall ( stack 2layers) which will give you more than 100 servings  4) It is not as hard as you think to smooth it:)  Use a long  off-set spatula or 18” ruler or yardstick (clean /new!)  

This is not two half sheet cakes, but it worked out about the same size as a full sheet cake.  I made the cake board from plywood and I smooth the top with a long, medal ruler!  Betty taught me that trick years ago and it works a charm!

  Wilton sheet pan 12x18  - 1 layer- will mark off into 54 2x2 servings. My full sheet is 18x26.

another idea for a non-flexible and lightweight board is the half inch foam core board --