First, how in the world do you all roll out fondant to cover 14, 16, 18 inch cakes? Do you have a huge roller and a huge counter? How do you then pick it up and place it on the cakes where you need it so that all sides have enough fondant to cover?
Second, I know you need to roll it larger than the total of your cake diameter plus the height of your layers (two times). I have found if I have too much hanging down, it starts stretching and breaking down at the top edge. How much larger do you roll it than your pan measurements?
The largest cakes I've covered with fondant so far are 10-inch and I'm having a really hard time thinking about doing larger cakes for a wedding. I'd appreciate all your tips and advice. Thanks.
Covering really large cakes with fondant is a lot of work. You can use a piece of string to measure how big the fondant has to be to reach from one edge over the top to the other. If you tiers are 4" deep you add at lease 8" to the size of the layer.
I roll fondant on a plastic sheet, so I can use that to transfer the fondant to the cake. It helps to have a second pair of hands.
I also won't do fondant for larger than 14" I suggest satellite cakes or buttercream for those.
I do the same as JoAnnB...use a large plastic mat, lightly greased with Criscoe. I roll out and flip the fondant several times while it is still a smaller size and manageable. If you are having sticking problems, lightly dust with constarch/powder sugar mixture. Then, I roll out to the size I need, carry the plastic mat to the cake and flip over on top the cake. Gently peal the mat away from the fondant. Good Luck if you try it!
You may need to roll your fondant a bit thicker than you are. Also, try not to overwork the fondant...start at one area, gently lift out from sides as you press fondant onto the cake working from top to bottom. You don't really want to "rub" a lot as that may cause stretching. If you measure right, you won't have much extra at the bottom of the cake.
One other thing you can try is encouraging the fondant to adhere at the top edges before you start fitting the rest on the cake. Sort of lift and pinch it ove the corner, not enought to cause a wrinkle, but enough to keep it from pulling down too hard.
I make sure I use enough crisco and do the keep it on the map and flip it technique. I actually do that on all my fondant cakes. I think it is much easier to position and center fondant that way.
Ancient Chinese Secret: Get a 24 x 24 piece of medium-weight vinyl (or that spiffy new Ateco blue silicone mat and cut 12 inches off the end), lightly grease with Crisco, and roll out your fondant. To figure out your diameter, add your cake size (example 14 inch), plus the sides (if two 2-inch tiers plus filling, say 5 inches x 2 sides) and add all that together (14 + 5 + 5 = 24 inches.) Roll out a circle that wide and LEAVE ON YOUR MAT. Put your arm under the mat with the fondant still on it, put your cake up on a coffee can to elevate it and put the bottom edge of your fondant (on the mat) flush with the bottom edge of the cake. Line it up, take a deep breath and flip the fondant mat over the cake. Peel back the mat and use fondant smoothers to smooth the top, then gently smooth out the sides. Works every time! 
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