I had a chart for servings I got from school that my chef made and that is what I use. It's pretty accurate, but I've tweaked it out a bit. Anyhow, according to that chart, a party dessert serving is 5 square inches. I think (for me), 4 square inches is good for a "party" slice. When I think "dessert" portion, I think of the cake being the full dessert. When I think "party", I think there is other dessert- scoop of ice cream, or what have you, and that 4 square inches (twice the size of a wedding portion) is enough, especially if your cakes are the same height as a wedding cake tier.
Wendy, I think what mbcakes meant by charging the same for birthday and wedding cakes is that if there both cakes are exactly the same, both tiered and intricately decorated, why should one cost more than the other just because one is for a wedding. (Am I right mb?) I charge the same amount as a wedding cake and tell the customer the servings are based on wedding servings and if they need bigger servings, then they need to order more cake. But in this situations, if they are getting a tiered cake and paying a lot of money, they are having more of an "event" than a backyard barbeque or informal get together at Great Aunt Mary's house. So there are other desserts as well (petit fours, minis, etc.) and all they need are wedding portions anyway.
As for smaller birthday cakes, using a 9" cake as an example, mine start at $55. (I don't do plain cakes; all of them have gp flowers or some kind of doo dad on them). One cake I recently sold at $55 had 3 smallish-medium gp roses on them, a quilted pattern going around the side (using a press, very fast), and a tiny gp flower at each "X" using an ejector (also very fast). Looking back, I should have charge a little more, but oh well, next time I'll get it right. Anyhow, I was thinking at the time, "Geez, this could easily be a tier in a wedding cake. And I would get $95 out of that!" I started to feel like I let myself get ripped off, but then I realized that even though the cakes are the same, there is a lot LESS effort that goes into a 9" cake and a 9" cake that is a part of a tier. For me, a 9" cake means a phone call from the customer wanting a cake, a 5 minute conversation, usually ending with, "Just make it pretty" or "do it however you think it will look best", and me making the cake, and the customer picking it up. A 9" cake that is a part of a tier means it's either a wedding or some other big important event or function. It means meeting with her for about 2 hours. It means letting her taste samples. It means me running around looking for the tools or whatever it takes to make her dream cake a reality. It means waiting for her to sign a contract and me really trying to sell myself. It means a mental bride who needs to call and email me several times or a million times. Basically, it means TIME and time is money. So keeping all this in mind, suddenly the $55 bucks I was making seemed like a lot more since it didn't require much time.
Mike (Mike's Amazing Cakes) has a "base" price for his birthday cakes then has additional art charges. I thought about using this method, but I am not a bakery and I don't want to offer plain cakes simply b/c no money is made from making them unless they're done in a high enough volume (you know, "Happy Birthday" with buttercream roses). So I was thinking that if there is any hint at all to a "base" price for a "plain" cake, people will see that and think, "Oh, she does plain cakes for $35! I'll just get a plain one and save money!" Anyhow, Mike has some pricing info on his site you may want to check out: Mike's Amazing Cakes