I'm making my friend's wedding cake in August. She's wanting 3 tiers of different flavoured cake covered in just buttercream as opposed to fondant which I can do. However she's also wanting rocky road involved, I think that the buttercream on the outside of the rocky road to make it look the same as the other tiers will make it too sweet and messy for people to eat.

What size fridge bake would you recommend going to and what could I cover it in without it being too sweet and avoiding fondant?

Thanks

Why in the world would icing a cake in b'cream make it messy for people to eat??  Here is a great b'cream recipe: https://www.cakecentral.com/recipe/22469/2-icing

Now as to it being sweet.........well that is the nature/reason for icing.  If your client does not want sweet maybe consider using swiss b'cream. To me it is just like eating a stick of butter straight but many people on this forum seem to like it.  

I do not understand your last ?......"What size fridge bake would you recomment".....this makes no sense.