Right now I am confused and don't know how much to charge for a wedding cake, the bride wants 3 small cakes for a total of 100 people. Very simple cakes only covered in fondant and fresh flowers on top (she is using these green cakes as a reference). How big should the pans be? Or better how much batter I should use? And how do I charge for it now that probably the slices will be smaller?

Well, I charge $4 per serving for fondant. Plus an equipment charge (to pay for hte cake drums, and you've got one tiered cake in that pic.) Plus a delivery fee. But YOU have to decide your pricing based on your market and your costs of doing business. That may not be the same as my costs. For example, I buy cake flour in 50 pound bags and I get a good price on it.

You're free to use any sizes that add up to 100 servings, round or square with that setup. Servings sizes and batter amounts per pan can be found on the Wilton .com site.

I go with the 1"x2" slice for wedding servings. I thought about going bigger, but I've served cake at a few occasions for friends and no one wants a piece bigger than that after dinner most of the time anyways. That seems to be about the size that most of the caterers in the area cut too.
Your pricing should be based on a few things:
1) The cost of your ingredients and supplies.
2) Time required to create cake & decorate it.
2) Your local competition or what the market will bear
3) Your skill level, especially compared to the other cake vendors in your area. Be honest and objective when considering this.

There are many other things to consider, but these are the biggest, IMHO. Good luck!

Thank you all for your great advice.

And let's discuss skill level. If you hae been decorating for 2 months, but can decorate like i's been 2 decades, then you charge full price. Don't get the "But I just started. I can't charge that much". Yes, you can. Go ahead and start low & see how hard it is to raise you prices when you feel your skill level has improved. Now, if your skill level isn't up to par and your cakes reflect that, well, I don't think you should be selling cakes anyway, so the price doesn't matter. Post pics. when your done thought cause I think those cakes are really cute!