I have been searching online and found so many different variations - does anyone know a good serving size guide that will tell you what its based on i.e. 1x2 slice, etc. as i know some of them have listed.

any help would be greatly appreciated!

Wilton's serving sizes are about the size of your first two finger (width and length of them). You can go to their website and look up the size pan you are using and it will tell you how many servings. (Personally that size serving just makes me grumpy ;P) Of course if you are doing a shaped cake that won't do a whole lot of good...

if you cakes are being cut by caterers, they usually follow wiltons guide, so it makes sense to do that your self too.
there are other guides, like earlenes, which give a more generous serving. although if someone then cuts it according to wilton, it looks like you sold more cake than the customer needed, so it can be a good idea to send out a cutting guide.
xx