Here is Wilton's serving guide,
http://www.wilton.com/cakes/tiered-cakes/serving-amounts-200-300.cfm
And here is Earline's chart,
http://www.earlenescakes.com/ckserchart.htm
You can choose what ever the easiest to understand.
HTH. 
those charts are confusing for me. and they seem on the small side. I want to make sure there is enough cake.
I have only done 1 wedding cake before and it was a huge cake for a small amount of people, so i am nervous now.
any ideas on cake sizes for 215 people?
People hear "one inch" and they think "paper thin". It's not. I use the wilton chart (1x2x4) for every cake I cut, be it a wedding cake or my grandkids' birthday cakes. It's a perfect dessert size piece of cake. It's not a Jethro Bodine size cake.
Here's a pic of a 1x2x4 piece of cake: http://www.cakecentral.com/modules.php?name=gallery&file=displayimage&pid=1156785 When I cut the wedding cake, I frequently have people asking me for "...a smaller piece, please."
Here's my website page on how to cut the cake to achieve this serving size: http://cateritsimple.com/_wsn/page10.html Same method for square cakes as round cakes.
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