I need to make a square groom's cake for around 150 people. The bride asked to make the cake for 100 servings, about what size would that be? Is it the norm to make the cake servings lower than the people expected to come? TIA!

There's a not insignificant price difference between 100 and 150 cake servings so I would clarify how much groom's cake the bride wants to pay for before doing anything else...

Additionally, the groom's cake is usually in addition to NOT instead of the traditional wedding cake - so it doesn't need to serve ALL the guests.

HTH