I need to make a groom's cake and not sure what size I should make it.
There are about 90 people expected. The wedding cake is a 3-tier (12, 8, & 6).
Isn't there a rule of thumb that you go by when deciding how much groom's cake you need?
What is the couple expecting? I wouldn't think they'd want a groom's cake the same size as the wedding cake, would they?
I do groom's cakes for $25 ..... a simple 2 layer 9x13 or 10" round. (BTW, this price is "when purchased with a wedding cake"). So far, my brides have had a groom's cake as just a side cake, not intended to feed the whole crowd.
If they'd want a larger one, I of course would do a larger one ... with a larger price! 
This might help:
And just where do groom's cakes fit in? The groom's cake, now making a comeback is a gift from the bride. It is displayed next to the wedding cake and sometimes served along with it. It can be made in different shapes, a popular being an armadillo. (from www.baking911.com)
http://www.thecakery.com/groomscake.html
I always leave it up to the bride but I always suggest that we never go bigger than the wedding cake especially in height. Not everyone will be eating a piece of both so I think it is up to the bride. I charge $2.00 per slice for a groom's cake and reall enjoy doing them because you can get so creative!
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