I have never covered a cake with fondant, and someone asked me about covering a sheet cake with it. That's weird right? How much would it take for a 12x18 (1) layer sheet? Could I even roll it out big enough??? Do you charge extra for fondant covered cakes?
How tall is the cake going to be? 2", 3", 4" ?
I roll like this:
if it's 3" tall you need a rectangle, one direction needs 3+12+3 plus 1 inch extra which totals 19"
for the other direction you need 3+18+3 plus 1 inch extra which totals 25"
You're talking almost 2' by 2' at 1/4" thick. I make MMF and to get that I would probably have to make like two batches to mix together. I'm thinking about 5 pounds.
Just see if you have the space to roll something that big, and enough fondant to do it.
As for me, I don't charge for cakes, but judging by the answers I've seen on here it looks like everyone does by the serving whether its fondant or buttercream, but don't hold me to that.
That's just my input... maybe a pro will reply with a better answer