Can anyone give me some help on letting people know about serving sizes, for 8, 10, 12 and 14" cakes. thanks. These will be only birthday, anniversary cakes etc as im so new Im not doing any weddings (although i hope to be able to one day). thanks a lot. Im just trying to price my cakes too for different recipes, styles and sizes. Any advice for a newbie in this area as all my work in the past has been for friends and now other people want to order and buy and im not sure what to charge to make it reasonably profitable.

http://www.wilton.com/wedding/cakeinfo/cakedata.cfm

A serving size according to Wilton is 1x2x4. That site will give you a bunch of info per cake size. These size cakes are 4 inches tall with filling. However, since you're doing party cakes, you could make the sizes bigger.

The Wilton site also has charts for serving sizes for party cakes. These charts are industry standard.

i use earlenescakes.com serving chart...the pieces are a little bigger than wiltons. go to the site, then scroll down and click on the right at "cake servings".

What the others may not be aware of is that serving sizes in the UK are considerably smaller than in the US (nor are the cakes as deep with the average being 3in). The wedding serving charts mentioned above will give you what a party serving would be in the UK.
As for making sure your cakes are reasonably profitable. First add the cost of all your ingredients (down to the last gram!), plus the cost of your board, box and any other sundries you have to purchase to complete the cake. Then estimate how long you think the cake will take you to make (including all the mixing, baking time, decorating time and cleaning up time!), and times that by the amount of money you would like to earn per hour and add it all together. This should be the price of your cake, and not a penny less! So, for example, cake costs £15 to make, and you expect it to take you 5 hours and you want £8 an hour for your time - £15+£40 = £55. Never ever undervalue your time & talent, Way too many people here do that IMHO!

Hope that helps!