Before I can determine the size of your cake, I need to know how many guests you are expecting. This is usually for a Wedding Cake purpose.
A cake is usually served as Coffee or Dessert. My standard tier is 4" high.
Coffee: 1"x1"x4" (2.5 x 2.5 x 10 cm)
Dessert: 2"x1"x4" (5 x 2.5 x 10 cm)
The actual cut size may vary, depending on how big the person cuts. This is just a guide.
I also quote less than the expected guests. Most of the time, many guests leave early or some don't even eat the cake. This saves you from wastage.
Below is a guide of cake portions based on the size & shape of a tier.