Don't mean to be contradictory, but since you said you wanted a guide specifically NOT for wedding cakes.....here's a link to the Wilton guide for party cakes, which are slightly larger servings.
Thre are different serving guides out there. Earlene has a serving guide with somewhat different numbers:
and her cutting guide is a totally different plan than the Wilton. She still is figuring wedding cake sized portions though.
I don't know about where you live and what the cake eating customs there are, but even though these pieces all sound really small, and I'm always afraid I won't have enough cake, there always seems to be MORE than enough. Here, at least, pieces are getting smaller and smaller and everyone is on a low-carb diet and not eating cake....
Explaining ahead of time what a standard portion is supposed to be, and then sending along a copy of the cutting guide is a way to show that you made the "proper amount". If, when you explain it when the person is ordering, and the person doesn't like the sound of it, you can suggest a larger or smaller cake and charge accordingly. I hear some people make styrofoam or cloth cake slice samples so the customers will know how big a piece is.
Probably way more than you wanted to know.... Sorry, I got carried away...