Here is my favorite serving chart(s) there is a few different ones around (round cakes, square cakes, etc)
http://cakecentral.com/gallery/1300478/round-cake-cutting-and-serving-guide
http://cakecentral.com/gallery/1300481/sheet-cake-cutting-and-servings-guide
http://cakecentral.com/gallery/1300479/square-cake-cutting-and-servings-guide
Since I use both party servings (2″×2″) and wedding (1″×2″) for my cakes I love having all the information for both listed along with pictures on one easy picture.
Nikki, So Cal, www.Facebook.com/nikkibelleperchecakes