Short answer - there is no single answer....
Depends on
1. who cuts it [experienced?]
2. type of cake - layers/filled/iced/decorated or single round/iced/decorated or rich [mud cake, fruitcake]
3. dessert substitute or cake 'n' coffee?
single round/iced/decorated - 1 1/2 x 2 in
layers/filled/iced/decorated - 1 in. x 2 in. by two layers high
rich [mud cake] - 1 x 2"
rich [fruitcake] - 1 x 1" [fingers - not slabs]
For cake decorators, the definitive alternatives -
1. Wilton's Cutting guide
http://www.wilton.com/wedding/cakeinfo/cuttingguide.cfm
2 Earlene's Cutting guide - 1" x 2" - 22 servings per 9" round
http://www.earlenescakes.com/cakeservinst.htm