Ok My girlfriend wants me to do a dummy wedding cake and have sheet cakes to serve for the guest. So my questions is what is the largest pan I can use to serve 150 guest? I did not know if there was an idustrial pan for bakeing large amounts. Also have long will the dummy cake last (how far in advance can I make it and it keep the fresh look? ) Thank you for any help or advice you can share.

You can make the dummy cake as far in advance as you want.
You'll want to keep it out of the sun light though (expecially if you have purples, pinks, or blues on it) to keep colors from fading

There may be a large pan to feed 150, but where are you baking the cakes? If it's in a home oven then the pan won't fit. Typically i have found that a 12x18 is the largest pan a home oven can handle. so using that.

12x18 that is 4" high (which i recommend for weddings, we call these kitchen cakes) feeds 108. So you can bake a total of 3 12x18 cakes. Stack two for the 4" height, then cut one in half so you have two 9x12 and stack these two cakes for the 4" height. This will get you a total of 162 servings.

Thank you so much for all your help