I am making a wedding cake for 140 - 160 people, but the couple only wants it to be 2 tiers. Will a 14'' and 16'' be enough or will there need to be sheet cakes too?
I think Wilton website has a chart for servings per what size cake
12 & 16" rounds will yield 156 wedding servings, while 10 & 16" rounds will yield 138 wedding servings.
Here's the link to Wilton's wedding cake servings/cake preparation charts for 2" pans:
http://www.wilton.com/cakes/making-cakes/baking-wedding-cake-2-inch-pans.cfm
HTH
I have no tolerance for brides who tell me they want 1000 servings but only want a 3 tier cake. (Big sigh!!)
I have no problem telling them, "Well, that's going to look pretty silly, having a 10" and 16" cake. You can easily have 3 or 4 tier cake for that many people."
And I HAVE told brides, "You can't have a 2 (or 3 or whatever) tier with that many people. Here's what we CAN do for you ,....."
That's like telling a home builder, "I want a 6 bedroom house but I only want it be a total of 900 square feet." 
Unless they are well versed in cake design (or home design) I dont' see where they get to call the engineering shots.
I have no problem telling them, "Well, that's going to look pretty silly, having a 10" and 16" cake. You can easily have 3 or 4 tier cake for that many people."
I always wonder if the brides that request 2 layers when more would be aesthetically appealing, think that there's an additional charge for having additional tiers above the per slice cost....
So limiting the cake design to two tiers is a way to save money on their weddng cake???
Like saving money by having a really small wedding cake with numerous kitchen cakes in the back....
Or a huge 5-8 tier dummy cake and tons of sheet cakes in the back...
Wedding cake myths and misconceptions. 
Here's a thread which illustrates this:
http://cakecentral.com/cake-decorating-ftopict-670348-.html
HTH
Jan, I never thought of that! They may be thinking they will save money by having fewer tiers. Poor things! All the bad info they've been fed by those terrible bridal magzs!
indydebi, sorry to highjack , but i have tried to figure out if your the person that shows how to cut cake? And if so what tool is it that you use. It look's like a metal wide tooth comb, of some sorts, would love to have one but have no ideal what the correct name is for it. 
I recently read in another forum a caker who refuses to make a top tier any bigger than 7" simply for aesthetic reasons.
To big tiers just end up looking flat and squat. Who wants a cake that looks like a football field?
Jan, I never thought of that! They may be thinking they will save money by having fewer tiers. Poor things! All the bad info they've been fed by those terrible bridal magzs!
indydebi, sorry to highjack , but i have tried to figure out if your the person that shows how to cut cake? And if so what tool is it that you use. It look's like a metal wide tooth comb, of some sorts, would love to have one but have no ideal what the correct name is for it.
Called a cake cutting comb, a cheese comb, a cake breaker, angel food cake comb. Can be found on ebay all the time.
Reply to hijack... sorry for that...but:
Yes, Debi does show how to cut cakes. And it's all with pictures and such:
http://www.cateritsimple.com/id10.html
Excellent!
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